I have a Z400 series as well; the kits are pretty nice and easy. However, you could have a fab shop cut you a few rectangles of 1" plate steel with a plasma cutter in dimensions that would work well on the caster legs, then use leaf-spring u-bolts to clamp them on. The metal shop could probably pierce holes in the plate for the u-bolts to travel down through. Chamfer the edges of the metal plate on a bench grinder, then paint the plates and u-bolt either orange or black, and bolt them on. Not sure if that would end up being cheaper than the pre-made kits or not. Overall, I've found my Z421 to be more stable on slopes than most zero turn mowers I've used, thanks to the fuel tank being under the seat and the low-profile tires.