Have you tried just leaving your bag unzipped and see how it works for you?
I've been elk hunting this last week (no luck) and did just that. I used my old rectangular down bag and zipped it up to a bit under knee high. We were in a camp trailer but left the heat off most nights. It would get down to about 45. My bag started getting cold about there, which is why I started this thread. Using it as a quilt worked great. I like it very much compared to a zipped up bag. The bag is 60" wide but I think a 55" would still give me plenty of width. I'm a side sleeper but keep my legs pretty well stretched out. I had 1 side well tucked under me. A couple pad staps on 1 side would just be a bit more insurance.
