Many methods. Where the monocote edges meet the firewall, run a little thin CA along the edge. No peeling. You can also treat the area where the edge will sit (before covering) with Balsarite, that helps too. You can also "paint" the edge with clear paint or dope. This seals the edge preventing fuel from getting under it and peeling. I'm sure you will get other suggestions here as well. As suggested, I would not monocote over the firewall, just up to it. Some people may have a good way to keep monocote on the firewall, but other than glue under it I don't know. I usually just paint thinned epoxy or polyester resin over it to fuelproof it and run the monocote up to the edge of the firewall.