Glenn, Yup, it will fit. The pad keeper will fit anything up to about 2" thick, maybe a bit more, with some persuasion. I have used 1,2,3,4 and 5 layer NightLight pads. But, I had purchased 3 about 10-12 years ago. This was the old style nitrogen processed pads that eventually get a bit thin. Personally, I don't think I would use anything that soft in the pad keeper, though. It will tend to roll up too easily. You might get away with sticking a couple carbon arrow shafts (grind down the ends to round them) in the folds with a bit of tape in the bottom to make some pockets.
I like a 2 to 5 layer NightLite for doubling as a frame/pad. 4 layers is just under 2", though I cannot say about the new stuff...it's also a lot heavier. The original 5 layer pad was about 9-1/2oz or so after taping, 51-52"long, and 3/4" thick. (Two layers, nested makes it come out about 1" or so.) Nunatak sells a Luna pad that can be cut down to fit and taped together. Or, you might just buy 4 more sitlights, but I am not sure about the nesting. I fold it to get it in, working it into the pockets, at first. After 6-7 times, it loosens up enough to just slip in easily. The mesh is fairly forgiving of being stretched.
I origonally used it with the old MiniPosa, G5, then the Murmur. When it is <40F I also carry the NeoAir. (And, more and more lately, old bones...) Makes a good seat leaned up against a tree or rock...nice for lunch stops. It also serves as a good frame for anything up to 25 pounds, but it does take up strap length. I would say my old pad has 2000-3000 miles on it.