What shape is the pad? I have one of those mummy shaped self-inflaters and slide it inside the sleeping bag. Since the cover is nylon then it feels like the inside of the sleeping bag. I did find I needed some shoulder insulation so I used a 1/8" 60x40 pad under the sleeping bag.
Since the Troop doesn't have another trip until January, I'm going to break out the machinery and see if I can mangle an underquilt together. I used Dejoha's poncho liner idea and it works well. I'll add a bit more insulation and turn it into a sewed up version.