I have never been able to stay on a pad or air mattress so I switched to a sleeping bag that has a pocket system to lock the mattress in. Big Agnes has some but I just love my Nemo Jazz (may be discontinued but you can sometimes find them in REI Used Gear) and have both an Exped Megamat and a REI Camp Dreamer. I also have an REI sleep system. Any of these combinations make sleeping more comfortable and keep me from sleeping on the ground with my mattress spit out of the lean to. You could try to make your own sleeve or straps to attach it to your bag as well.
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