I used to use, and still have, a square section of blue closed foam cut out from an old blue sleeping pad, that I woiuld throw into my knapsack when going out cross country skiing where we would stop for lunch or whatever. It weighed nothing but you wouldnt get a cold or wet bum sitting on the snow. I actually had cut it out originally as a back protector for when I used to go karting at these indoor karting places, and the seats would hurt my back, so I took that and it helped a lot with not getting a bruised back from sliding around against the hard seat in corners. Fun as heck, but pretty expensive.