Good tip! Some packs are designed to handle a frame sheet externally to the pack. I think Gossamer Gear inovated this with a few others picking it up. Using a modified closed cell foam pad can net a good weight savingings this way. A full Nightlite pad can be cut into 10" sections, nesting the dimples, and taped together for a 50" pad/framesheet.
With a little modification, closed cell pads can be made into a box in a pack's body. A piece of duct tape will tape it back together, once you have the pieces cut to fit in your pack.
Example: 12", 7", 12", 7", 11" will give a 49" length pad. In a pack, this will give two layers of 1/2" foam back, a 1/2" piece of foam along each side and a piece across the front. This is more rigid (by a good 50%) than a simple inflatable framesheet and less weight by a couple ounces. It really holds the pack body stiff for up to two weeks of food (22lb) and your base gear (around 10lb.) Fuel makes up the 35lb maximum.
I have found that the NeoAir series do not lend themselves all that well to frame support, though. A more standard Thermorest works better, generally. None of the true inflateables really do a good jod with support(Pacific Outdoors, Inertia, X-Frame, etc.) I never tried the DAM's though.