Good idea - skate wheels.
(Also available on McMaster, for the impatient [me].) I replaced the skid pad while I was at it, just in case. These are rubber-on-steel 100lbs wheels.
I also wanted to make a rubber boot to replace the duct tape for weather-proofing the hinge tube tops. (The two outside tubes are welded closed on the bottom, so water can fill the tubes if it can get in.)
I wasn't happy with the strap hooks on the trunk lip - minor inconvenience for trunk access, and the straps were pressed against the trunk which caused paint wear. Just one layer of steel panel to drill at this location:
Use a sandwich to protect the car's sheet metal: anchor / neoprene / car-body / neoprene / washer.
(Also showing minor improvement; using velcro to secure rubber (gap filler) to the car. I'm a little bit lazy so I only added this at the top where it was most needed.)