Using 5 minute or 30 minute epoxy, thinned about 25 to 50% with rubbing alcohol works well and will last longer that the plane probably will. I have a couple of planes more than 8 years old and the epoxy is just fine.

You can also paint the firewall with a urethane bases paint. Works just as well. You can cover over either type of finish if you wish to.

Make sure that all the openings in the firewall that you are going to need are already complete and in the right place. Place a small piece of masking tape over your blind nut openings to keep the threads clean.