Here's three different makes. Maybe you can look at the pictures and get some ideas on how to fabricate your own...

http://www.springtie.com/

http://www.redwolfranch.com/highline-trailer-tie-grey-p-5.html

http://www.tilt-tie.com/

 Then there's the "Easy Care" product..

That's alot of money for a piece of pipe, a spring and some bungie cord!!!

The HiTie product is made from a fiberglass rod. They claim the rods are used to moor boats in slips. So maybe you could find them in some kind of Marine suppy center. They are 1" in diameter at the base and about 4' long.  On the fiberglass rod is attached a loop on the terminal end for you to attach a lead rope. And on the mount end is a hinge assembly that allows the unit to fold 90* so you can fold it in tight against the trailer for travel and fold out to tie the horse to. All metal is stainless steel so there is no rust or corrosion.

The fiberglass rod is strong enough that normal type of tugging by a horse won't break the rod. But it has give so it will buffer the tugging of the horse through the lead rope.

I doubt you will find the needed hardware to make one yourself. It is probably custom made just for HiTie.

If you want to make a home made product, you will need to decide just wat features of the HiTie are important to you.

I'm sure any competant machinist could fabercate a metal bracket to attach the product to the side of the trailer.  As mentioned the original product used a trailer hitch reciever that was bolted to the trailer and modified stinger was dropped into the reciever and a bolt was pushed through to lock it in place. The reciever/stinger are usually steel and will rust, are not very light weight and don't have the pivot feature of the newer versions.

Add to the stinger some type of extension rod to hold the lead rope out away from the trailer. The very first products were a solid bar or pipe. But this doesn't flex and transfers too much of the horses movements into the trailer frame. Especially if you are trying to sleep in the trailer. Next generation models included some form of Spring to dampen the movements and to put subtle pressure on the horse to ask for a give to pressure when the horse reaches the end of the lead. Some of these features could be had by using a bungie to attach the horses halter to the rod. Remember what ever spring or bungie you choose needs to be strong enough to support the weight a horse will apply.  I personaly like something strong enough so that when I tie the horse's head 6-8" off the  ground, the horse can flex the system enough to get his nose to the ground to pick up feed, But strong enough to prevent him from keeping his head in that position for very long.

I know that buying 3-4 HiTies at $300 a pop can incur a sizeable expense. I suspect by time you buy parts on a onesy-twosy basis, buy 2-3 versions of springs or rods of different strength to make sure your parts are strong enough and have the proper flex for the job, and spend your time to put it all together, you will be better off to just write the check and buy the product.  But this is America and people are known to be inventive and innovative. I wish you well with your project and let us know if you develop a much cheaper method.

As far as using the HiTie products. I've had them on the last two trailers I've owned and when I get a litte extra money, I'll install them on my current trailer. If you camp with horses and camp in areas where you have to tie to the trailer, and try to sleep in your trailer, they are a must.

Places like the desert have no place to tie a horse

My horse lay down at night after a CTR or Endurance ride.  They couldn't do that when tied to a tie ring on the side of the trailer.  I've even had them roll while in the hitie. I've never had a problem with them becoming cast or caught under the trailer. The 4' offset  that the fiberglass rod provides seems to get the horses out and away from the trailer far enough to avoid most poblems. Never have the horses pawing the running board or trailer side when I use the HiTie

I don't use the bungies that HiTie includes. I tie the lead rope directly to the rod ( like in the picture) One night I tied it just a bit too long. My horse tried to scratch his chin with his hind hoof.  He got a hind leg over the lead rope and we had a circus for a minute. I've been much more care full about the lead length ever since. Long enough that they can STRETCH to get their nose to the ground to pick up hay, But with enough tension that they won't hold it there for long.

If they lay down while tied they don't put their head flat on the ground. But rather sleep on their belly with their head upright. Like this photo of my horses on a Highline. In fact the elad rope is a bit long on the black horse. When laying down they should have their head up more like the grey, Then they won't accidently get a leg over the rope when they go to stand up. In this picture the Highline had stretch a little and the sag  lowered the lead rope. If they had been on a HiTie, there would be no sag.

No sag or droop.  At least not in the 5 years I've had them.

Being made of Fiberglass and always exposed to the elements on the outside of the trailer, The finish on the rods wears off and then you can get some fiberglass slivers in your hands on older rods when you handle them.  I suspect you could coat them with some kind of tung oil or varnish to seal the fiberglass fibers in. But I just put some gloves on when I handle mine and don't worry about it. The only time you ever touch them is when you swing them out into position or fold them up when you are done.

When I installed mine I placed a bumper on the trailer side that the end of where the rod rested against when not in use. In order to close my rods into the travel position, I have to force a little flex into my rods against the bumper before I put the pin in.  This keps some pressure against the bumper during travel and they don't rattle or scratch up the side of the trailer when bouncing down a washboard forest service road. Even holding them in this slightly flexed position, I have not noticed after years of use that they have warped

I would not recommend applying oil or varnish to the fiberglass rods. I'm not certain, but they may have been constructed with either epoxy or vinylester resins. With age, both are subject to UV, as well as mechanical damage. The best repair is a new applied coating of the same resin. The specific type can be probably learned from the manufacturer, and purchased from several fiberglass sources. The repair would not be difficult to effect.

Gard

Can you just sand the area smooth with some 180 -220 grit? Even an application of a tinted "boat yard resin" (polyester) would seal the area and replace the eroded surface. It's inexpensive and available in quart cans at many places. You can buy small tubes of various primary colours that can be added to the mixture, which will increase the durability from UV damage.

Fiberglass splinters are the worse. You can't see them, they burn and they stick inside.

Gard