It really depends on how long you want the walls to last, and Lexan is obviously the way to go, but you could also look into Mirror Foil Sheeting Contact Paper.
Personally I think the reflection abilities are about the same- and a roll of 3' x 33' is around $200, that could get you about 4 walls a roll. You could then support that over with plexiglass over it to protect it and switch out every year as necessary. The best part is that you could also do your ceilings with the same material to create even more confusion.