As FishEH suggested, and you can rig the base of anything to level at 4 corners, the screws on an outboard motor have a flange to fit against the transom and a turn knob, same principle. I've never used an RTT but it has a base platform and I'd use a threaded flange underneath the platform with the end of the threaded rod extending through the platform to adjust it. Perhaps you could offset it at the edge of the platform with a frame under the platform. It could go to the sides and fasten on as needed too. The longer the threaded rod the more adjustment you can get. LOL, I wouldn't try 3' of travel on a 1/4" rod either. Your washer and dryer have levelers, so does the fridge and freezer, just copy the mechanism the foot could be spot welded on the rack too and use more threaded rod. Need a quick adjustment, put a fixed nut on the top of the rod and use your battery powered drill.