I know many people here are into teeny-tiny kits, but for a larger pack or vehicle, I use a length of Reflectix insulation.

You know the stuff, the multiple-layers of laminated bubblepak and mylar film. It's a quarter-inch or so thick and you can often buy it by the foot in 18"+ widths.

You can lay on it and it makes a LOT of difference under a sleeping bag. You can sit on it in front of a fire (with something behind your back) and it keeps both sides warm at the same time, unlike the usual fried in front/frozen in back.

If you spiral it loosely around the inside of your pack, it will also help protect other contents from exterior bumps. If it gets a cut in it, no big deal.

Sue