Help Needed to Reduce Installed Generator Noise

"Thin" and "sound absorbing" don't go together very well. Adding structure to stiffen the panels so they can't vibrate as much could help. That is blocking transmission, tather than absorbing sound. A laminated structure of rigid foam sandwiched between two sheets of plywood or similar sheet goods is quite rigid (and strong) for its weight and might be one decent possibility. More simple is just adding mass, say with dynamat.