Husband just welded one up for our trailer, went full width across the back, fit on the brackets already on outside walls to latch in place with drop pins. I didn’t want the big mare leaning back on the rear doors, possible breaking off the door hinges on rough roads. This single, bigger (3") diameter steel pipe, worked just fine for her long travels. She used it when she needed it, stood nice when she didn’t need the bar.
I expect your local welding or fix-it shop could make one up for you pretty easily and probably not too expensive either. Make sure they clean off any welding spatter and the welds, so there are no sharp edges. Use the larger diameter pipe, it is much more comfortable and stronger, to lean on for horse. You can put a pad or cover on it if you like. Pipe insulation or split pool noodles are great covers when taped into place.
I liked my long butt bar to use. We also took out the divider for the big mare so she had more room standing at an angle.
Husband took the full divider and shortened it, so our smaller horses could splay legs out a bit if needed for turns or rough roads. I never use a full divider, best way I know to get a horse fighting the trailer in transit trying to stay upright. They turn into ugly loader and bad trailer rider in short order, even with pretty nice driving.
You might ask the Welder folks if they could NEATLY shorten your divider for you, what it would cost? Compare cost to buying a half divider from the trailer maker, so you can haul two horses at the same time in the future.