[Sunwapta]

A few thoughts...

One, has anyone tried using some sort of fabric stretched under their vehicle to smooth the airflow? (Are there any suitably stiff and fireproof fabrics out there?)

Maybe even sheet rubber with foil or something adhered to it where necessary. Or multiple 15' - 20' lengths of landscape edging (LE) attached to each other with metal strapping. These LE bands might better follow the undercarriage / suspension contours.

I imagine fibreglass fabric could be stretched out, attached, holes cut or spaces created for the critical heat and wear areas and then coated with the hardeners. It would likely require midway attachments or reinforcing to prevent it coming loose in the middle of the underbelly.

Two, this might only be appropriate for trucks and vans but has anyone used sheet coroplast or sheet metal to minimize all the cutting and fitting by building a drop frame and just covering the whole underbelly? You'd likely loose a couple inches of road clearance and still have to cut around the suspension and deal with exhaust heat but would this simplify the process?

Three, rather than filling the voids with coroplast or something, instead would two or three underbelly spoilers or airdams do the trick. (Or would turbulence negate their effect?)