Fyi on our experience with a commercial tray with 3 sides - looked like a good idea but Gracie didn't like it. She's used to a flat tray. she sometimes miscalculates and puts her front feet in the tray and her back feet out, so you know where the pee goes! We thought the wall thing would help, but I guess she feels confined (it's like 23" by 24'). Might work with a bigger area, though. Or with a single wall for boys. I like using disposable pads (or paper would be fine) as long as it can be clamped down to prevent the 'oh this looks like a big piece of kleenex, I think I'll run with it' experience
I also want something that protects carpet. Generally, 24" by 24" works for Gracie - as she gets older her aim gets better! Good luck on your design.