In reply to Chris Ellyatt:
so, a multimats inflating right. dont cut it down (like i first said till i realized and rewrote my post).
just yank the existing foam one out of the pack and replace it. or if you get a 3/4 length shove it in with it and use both.
the ultimate in space saving tho is chopping up a foam mat, not just into sections but also tapering it and taking off corners and packing it flat.
or one of those new Klimbit things, but they still need a foam layer to prevent cold patches on snow.