I made a frame using steel strips with pre-drilled holes (from Home Depot?). Since I couldn't get to the back of the frame when I put the belly pan on, I used speed nuts (aka spring nuts or U nuts, similar to these:
3/8"-16 Panel Range:.050"-.200" Center 3/4" Extruded U Nut | Fastenal) and small screws with washers, with Loctite blue.
Frame:
Belly pan:
Belly pan was on there for four years and never once had to do anything with it.