Heh I'm just waiting on some material because I changed the mounting design again.  
My newest plan is to get a larger piece of rectangle tubing and hack off one end.  Then I'll slide it over the frame from the bottom up and place two bolts in the top so it clamps tight.
My uncles shop didn’t have any tubing that was big enough so I gotta pick some up tomorrow from the steel recycling yard.
Got to be careful on the square tubing. The internal radius of the material may interfere with the external radius of the frame.
Have you thought of taking a square plate, welding tubing to it, then drilling the outside portion of the frame the diameter of the tube, and drill the inside of the frame the bolt diameter. Support the bolt head with a flat metal. Use 4 bolts, mounting in 3 places on the frame, and you should have plenty of strength. Then to add, make the interior plate with a flange so as to mate a skid plate to it.
Just my thoughts after laying underneath my XL7 yesterday.