might be a stupid idea.. but if the steel wall is thick enough.. maybe mount the handles onto a pair of 50 pound pull industrial magnets and just slap them on the sides. (only thing that might limit that is the steel thickness, as that limits the amount of total pull the magnets can exert. if you can get full pull on them though, then with 100# per handle, it's pretty doubtful they'd come off unless you're lifting the box crammed full of meat) if I remember right, the unit only weighs about 60 pounds, so.. that's an idea. another would be a pair of steel straps (16th of an inch thick should do) wrapped vertically around the sides, top and bottom, and the handles bolted or welded to them
hope that gives you at least something to kick around.
