Dave53, I use 2 old sheepskins 'bum to bum' together in the bottom- cos I'm an old fart (artistic license used on the name turboGIRL 
I use a foam pad under them too and they sit perfectly. Old ones are that bit felted so squash down better but with ma duvet they still make for a big roll.... but I'm a lazy woodswoman so don't tend to walk too far with it so not a problem! I've not had time to prototype a peapod yet (amazing how you don't bother until the weather kicks you up the butt) and I'm dying to play around with the 'snakeskin' idea too. I don't want to ask about them until I've had a good trawl of the forum n know what experiences are out there to learn from
I use a foam pad under them too and they sit perfectly. Old ones are that bit felted so squash down better but with ma duvet they still make for a big roll.... but I'm a lazy woodswoman so don't tend to walk too far with it so not a problem! I've not had time to prototype a peapod yet (amazing how you don't bother until the weather kicks you up the butt) and I'm dying to play around with the 'snakeskin' idea too. I don't want to ask about them until I've had a good trawl of the forum n know what experiences are out there to learn from 
