Fun, I used a MYOG Meteor and a 5x9 tarp for the AT. If you're tall, the extra foot in length is well worth the extra ounce even with a bivy. A few rainy nights it still felt short, but I didn't pitch well.
I used a bit less zipper on my MYOG Meteor, all at the top. It was never a real issue, but a minor inconvenience. To do it again, I would use the full amount. You can skip the tieouts on the bottom of the bivy.
One thing I changed was that I made the hanging string adjustable from inside the bivy with a mini cord lock. Makes entry and exit much easier. The string goes through a tiny reinforced hole at the peak and then through the cord lock. I store the spare string against the cordlock and netting with the pressure from the taught cord that's holding up the peak. Loosen before entry, tighten, loosen again for exit.
It's a shot in the dark, but you could save zipper by making a top entry design like the MLD Bug Bigy
Great piece of equipment. I'll be making something a bit roomier for the PCT though, like an MLD Serenity or HLM Gear Echo 1 Insert. Still going to use the 5x9. Best of luck.