[Phase]

Originally Posted by aerohead

1) Any existing holes can be re-purposed for attachment points.
2) 1/4- 20 all-thread can be cut, bent into a 'J' on one end, and slung over an existing rigid structure to create a tension member to 'hang' a subframe, skinned, and stiffened with aluminum angle extrusion, box-section, square tubing, etc.., 'pinched' against double-nutted fasteners, locked at the desired elevation for the panel.
Welded, brazed, spot-welded, bolted, self-tapping screwed, riveted, epoxy-bonded, etc..
You have exhaust heat issues, so that's got to be addressed either with custom heat-shielding, or the use of sheet metal.
10mm, fluted Coroplast would be stiff enough for non-heated locations. Throw metal at problem heat areas.
Spread compressive loads of attachment over wide fender washers, so as not to 'crush' the panel. Nyloc nuts cannot vibrate themselves loose.
Dzus fasteners are great! it your budget allows.

was thinking about underbody being abs sheet and then cut out the sections that are near the muffler and use aluminum there

bolting diffuser on would be easy with the attachment points underneath. just would need to keep it stiff and stable at higher speeds so it doesnt flop around