Are end caps an option? I'm thinking sil nylon, or maybe Tyvek. What about IX?
I use a weathershield to hold in heat above and keep moisture off below the peapod. I'd imagine that anything would work, but breathable is better. Draping a weathershield over my hammock's ridgeline also moves the frost out from the peapod onto the weathershield. (You just have to be careful about letting it drop on your peapod when getting out.