Keep the Thermarest. Find someone who buys table wine in those 4l casks. The bladders in those casks, once empty, weigh about the same as a ziploc bag of comparable size. Get the number of those that you need...probably no more than 2 or 3. Just hold the spout open to inflate them to the hardness you want and use them wherever. To pack out, just lean on them holding the valve open. They work great and will hold several hundred pounds each without breaking. They're light, compact and free. Best of all, they work.