If you build over the wheels, the way the Harbor Freight trailer is made with the bracket for the casters to attach to it just begs for some quick-to-install mud flaps. I was going to use actual ones from the auto parts store but due to the short budget I just trimmed and painted some lauan and stuck it on there with a rubber bushing to add a rearward slant to the thing. Cost: $0. It deflects road spray and actually doesn't look too bad. I think I will replace them with rubber or plastic some day but for now at least I have flaps.
Ed