To have an opinion (I am never short of those) I’d like to know the make of the trailer, or a general statement as to what it is: Livestock, two horse straight load, three horses slant load --ideally the manufacturer or photos of the interior to see what structure one could attach a center panel to.
As to butt chains v butt bars --my most recent trailer (I’ve had 8 if I count those I had only briefly to repair and resell) my most recent trailer: 2020 Merhow 2H straight load, has heavily padded butt straps instead of butt bars or butt chains. At first I was dubious but was assured they would replace if I wanted them replaced with butt bars. (I only live 7 miles from Merhow’s factory). Honestly, I would not go back to butt bars or butt chains. The padded straps fold up to let horse out --no clanking on the pasterns, and unlike chains, they don’t destroy the tail. Tried to find a pix but no luck . . .guess I could take one when I go out to feed today --if you are interested.
Another thought is --if it is doable --I have my trailer on my property and unlimited time to spend with my horses (retired) --is hook your trailer and leave it hooked (yeah, I know, pain in the butt but I don’t go places much) --then work your horse as usual. When you have finished. Load him. Let him stand (you can stand with him inside the trailer the first few times). After a couple of minutes, or 5 or so, but when he’s still calm, unload him. Ride some more. Load him. Ride some more. Load him. Do this for a few days (5 if you can) and increase the amount of time he is in the trailer between work secessions --it can be in hand, or lunging. Work toward leaving him the the trailer while you do something else. He can have hay.
The point is --trailer=rest. My horses come to view the trailer as a joyful place to rest. All of mine (4) are self loaders and quiet riders. Bob does paw, but he wears pawing bracelets to discourage that. I think he’s doing it less. I have also heavily padded my divider support in the front to prevent him from hitting it when he does paw.
Of course the trailer=rest is just about impossible if you don’t have access to the trailer/truck and the time to do this. On the other hand, even spending 5 consecutive days for an hour or so, might cure your horse.