Open cell foam is obviously a bad idea for ground sleeping because of the way it compresses and soaks up moisture, but it just hit me the other day that it might work well as an under quilt, so I started googling. I found the Supershelter, but I'm curious if anyone has done a DIY version. It seems the challenge would be to eliminate gaps between the pad and the hammock. If you could manage to do that, I think with a simple undercover (polycryo or e-blanket), and an e-blanket over the pad to keep out condensation, it could be a really good winter system for pennies on the dollar compared to down.
I have built an undercover out of emergency blankets, but it hangs really loosely, so it wouldn't work for this without fixing the way it hangs. I'm thinking I could just get one of those half-inch thick mattress toppers in twin size and cut it into three pieces. Then I just stack in however many of them conditions call for. The undercover would have to be fitted enough to remove gaps but loose enough not to compress the foam.