I've got this 5 cm self-inflator, and I agree...it's not the most comfortable for this old boy:
Designed specifically for campervans, the PopTop self-inflating sleeping mattress offers ultimate comfort for campervanners. Created from 75 Denier, soft, brushed fabric with 3D construction to ensure the entire surface area can be used, you can enjoy a comfortable night's sleep wherever you go...
www.vango.co.uk
But I don't leave my mattress up top when not in use...I deflate it every time (getting it back in the bag is always a great way to end a trip too
). I just can't bring myself to leaving it up there...am worried about lack of circulation/condensation, etc...I have a Hilo, and so there's zero amount of wiggle room once the lid is down...even though I know some do leave theirs inflated