1. I got another idea for a GPS antenna mount tray that will be mounted on the firewall. I dug around through my crap bin and found an old ELT mounting plate that I think I can trim to size. I even has a stiffener riveted on the bottom of it.
2. After a lot of futzing around, I decided the old ELT mounting plate is not going to be suitable for the GPS antennas -- ironically, because of the stiffener on the bottom. It gets in the way, and I would have to drill it out, which would look junky. So I threw it back in the scrap pile and began constructing a new mounting plate from scratch.