bolted mine on
Suz, I had to use a shim or another piece of square tubing as did Joe. I, unfortunately, can't weld. I'd love to learn. Lowes had some in stock. I think it was about $8 for three feet. Its exactly the same size as what our older Casita's used.
The outside frame is twice as thick as the cross bars. I just cut a piece and bolted it on first using self tapping bolts, or screws. Then it was level so the jacks would fit. I drilled larger holes on the bottom that the socket wrench would fit through. Not the socket itself but large enough for the shaft to pass through. I put the bolt through the top hole first. Then the socket through the open end. Then fed the shaft up through the bottom hole into the socket. I sketched here a ratchet but actually had the socket spindle in a drill. I also drew the new piece as if it was flush with the double frame. Leave enough space so you can put the socket in and get it back out. I hope this makes sense.
I'd take pictures but I'm dodging tornadoes at the moment.
I've ordered a lot of stuff from JC Whitney and returned some as well. I haven't had a problem but I don't go back 20 years. I got them on sale for $46.