At our beach we have a home made cat dolley which is free to use for everyone. Welded a square tube aluminum base with a 9' handle coming out the front. For ties we found some huge ATV wheels. A little carpet and a set of rollers... ended up costing $250.

I find it hilarious that you can go out and buy a new cat trailor for $400, but you will have a tough time finding a cat cart for the same price... I think the suppliers are making a mint on these things.