What about hot glue to join the XP to the fabric? Maybe just in 4 or 6 spots? Hot glue is flexible, and I would imagine you could push the XP down into a blob of it to secure it. Or, maybe you could apply the blob through the XP by pushing the tip down into the XP and onto the fabric. If hot glue won't stick to the heat sealable side well, maybe you could add small round patches of sealable, so you're working with the non-sealable side for the hot glue spots…? Or, maybe you could heat seal thin strips of the sealable stuff in 4-6 places on the bottom fabric, poke those through the insulation, and heat seal them to the top piece, so that the insulation is secured in several spots…?
I don't know anything about the fabric or the insulation, so just tossing out ideas. I am really interested in this thread because I'd like to see an insulated air mattress that is as comfortable as my big fat Thermarest, but at maybe 1/4 the weight. 10-11 oz. for an insulated, 1.2" thick air mat sounds great. Or, even better, a 2.4" pad for ~ 1 lb.