I've slept in the heatsheets bivy several times (with much condensation, don't recommend it) and realized that I don't like the end above my head open as it is difficult to close off the bivy to protect from drafts coming from behind your head down into the bivy.
So, I taped the two short sides (head and foot) completely, and taped the entire length of one of the long sides. On the last long side, I taped about 3/5 of the length and started from the corners but left an opening towards the head area. So there is a footbox and a headbox, with the entrance in the center.
Also, I left about 2 spare inches down the partially open side so that I can tuck the bivy in around my head if it is cold or clip it up if not. In anycase, I prefer this because you have protection from behind your back, above your head and below you, and if you get drafts from the section open, it is easy to manage if you lay on your side and hold the bivy with your arms. Since I'm a side sleeper, this works best for me.
The polycro ground sheets are 40 inches wide and this is almost exactly the right size for someone using an inflatable 2.5" mattress....if you sleep on something smaller, then you could go with a smaller width.