Ah... just forget that fairing bit... it's purely cosmetic. IMO the ideal look is nice enough to not attract cops and ugly enough to not attract thieves.
Turbothrush built a very nice looking camper with pink XPS foam and hand lay up. Some photos here:
https://expeditionportal.com/forum/...r-hard-side-camper.58561/page-38#post-2814627
He used a dog brush to score the foam. I had the idea to use a spiked wallpaper roller to make many small indentations, fill them well with epoxy, then lay on the cloth before the epoxy fully cures. Should improve the bond and crushing strength of XPS a good amount.
I switched to 1.5" PVC foam, plus I have much more robust skins (>30oz cloth on each side, epoxy). Still plenty light in the grand scheme... ~1.5 lb/sq ft. Not that cheap... but at least I got most of the epoxy from Ebond before they raised their prices from ~$28/gal to $48/gal.
That might be a better way to go... but the issue I had with that was bubbles in the holes. I was doing hand layup with XPS and epoxy at the time...
Isn't polyiso even weaker than XPS foam typically? How good is your bond between the panel and the resin?