Why not make it cinch-able like a quilt? No matter what its going to require a bit of extra material if you want to do a fold or cinch etc.
You mentioned doing the top long, would that mean your feet would be on the ground? Or are you thinking about keeping it a really long bivy sack?
I would worry about rolling, I'm a side sleeper so I roll a lot personally. Are you planning to use stakes at all to keep the bivy sack stable? If not, I would be sure that your design is side agnostic.
I'm thinking of how I fold chip bags
, but I think it would require more material to do the fold than you have or want, idk. I would not enjoy being worried about undoing the fold in the middle of the night, especially if there are scorpions and snakes about!
Could add a buttons or install grommets.