As long as you don't plan on using the bumper mounted hitch to pull any real loads, use (2) pcs. of 2" square tubing x 1/4" wall thickness x 14" long in between the frame rails and the hitch mounts. Drill three hole sin each one to match the existing hole pattern. I would use Gr. 5 or Gr. 8 bolts 2" longer than the OEM bolts. Any time you modify your factory hitch like this, you risk losing some stability, esp. in the left - to - right loading. Square tubing is pretty good material for this, but you will need to build a "tight" assembly, with no slop in the bolt holes. Probably better than setting your slide-in on top of 2" of plywood though.