You can have Ron make your MLD eVent Soul Bivy with a full length, two dual sided pull zipper I believe. I think they’re running a 10% off Black Friday sale this week as well.
I will give Borah a call and see what they can do. The MLD has a full length zipper option but the price and cuben fiber floor concern me. If the Rab Alpine bivy had a little more girth and a longer zipper it would be about perfect.
Shameless plug, I'm selling my Rab Alpine bivy on here. It is eVent, the zipper spans the top down to just below chest height on both sides. Not full length side zip, but with some shimmying you could accomplish the objective.