I use a 48"x20" piece of firm convoluted open-cell foam, about 2" thick, with a homemade nylon cover. It weighs 23 ounces, and with a little effort, rolls up to about the size of a 3/4-length Ridgerest. The bottom of the cover is coated nylon, so I can lay it directly on the ground without worrying about dampness if I want a place to sit while I eat or just loaf. I have the convoluted side up, because it rolls up better that way, with the coated side toward the outside. It might be slightly more comfortable with the flat side up. It's a little bulky, and certainly not UL, but it's more comfortable than anything I've tried. The insulation is sufficient for cold ground, but not snow.

Others I've used: Flat closed-cell foam, Ridgerest, Z-rest, original thermarest, ultra-light thermarest, and the old Berkley AirLift air matress.