Have you ever tried leaving it unzipped and just using it like an overquilt with your feet in the footbox? I do that in my hammock when it's warm enough and it gives me more wiggle room.

If you go with a bag like the Big Agnes, with a pad sleeve on the bottom, and slide your pad in it, it may help keep the bag in place. I have slid my pad between my bivy bag and sleeping bag when cold weather hammocking, and keeps the bag in place better.

Good luck!