So Kristi and I are borrowing a Wildernest for our F250 (Thanks again Martyn) and we need a mattress or sleeping pad. We were thinking a Therm-a-rest but are finding they come in 77" length. We have a 50"x72" area. Any ideas or experience. I have back problems and a very bony hip, so I need to have cushion. Any recommendations would be great.