Okay, not exactly a solution for your trailer, but might work for the stall.
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I think that could work just fine in the trailer. As long as it was built sturdy enough and was either tight enough to the sides to not move around or was bolted in place or you used wedges to keep it tight and from moving. If I was doing it, I would use square steel tubing and weld up the frame, but you'd either need to be handy with a welder or know someone that is. I would either cover it with at least 3/4 plywood (not particle or chip board) preferably pressure treated for longevity or either pressure treated 5/4 X6 deck boards or a synthetic like Trex (all of these various ideas are budget based, Trex or equivalent being most expensive.) Then I would either paint it with good gloss paint for easy hosing and add non skid additive or sand (preferably course) for traction. You could also use epoxy garage floor coating (again adding either sand or the non-skid additive) or get either room on, spray on or professionally done bedliner, like rino lining, linex (I don't recommend it) Rustoleum, etc. All of these options come in colors, some tinted to your desired color or color matched to trailer.
The "extra window" would actually be a great idea, wouldn't look terrible, especially if you get a matching one from trailer manufacturer and either do a good clean install or have it professionally done. The thing about that is that then when you do sell it, it can be advertised as a "Custom horse trailer for minis AND/or full sized horses." Which greatly improves your attraction to all buyers, especially someone like me that has fullsized horses, donkeys and minis. That would definitely peak my attention! In fact, I'm going to order a new window (actually a couple) for my trailer as soon as I post this. I currently just allow my minis to "roam around" in my full sized, 3 horse bumper pull, which they do well with, but my mare really doesn't like not being able to see out, so she'll occasionaly rear up and look out when I stop.
Marsha, if you can, will you post the video of the mini in the back of pickup truck? I'd love to see it.
I've actually been considering either modifying my service dog, Angus', trailer that I pull behind my motorcycle for my minis or building a new one. He loves it and I'm sure they would too! If I knew how to post a picture, I would post one of the trailer behind the Harley with the 100 pound and 125 pound service dogs riding in it. Being that it has pulled those 2 and my friends 125 pound pitbull/American bulldog at same time, I should be able to tow my minis, one at a time, of course!