There are a lot of great ideas here - If you have an extra 1/4" of room in the plane I have a slightly 'Different' way to mount a blind nut.

First; cut a small block of 1/4" plywood about 1/2" square.
2. Drill a hole through the wood that will accomodate the Blind Nut,
3. Mount the Blind Nut in the block of wood,
4. Put a small amount of vasoline on the screw threads,
5. Drill the holes in the firewall as required,
6. Put Epoxy on the block of wood and align it with a hole in the firewall, insert a screw and tighten it down.
7. Repeat for the other holes as required.

The advantage to this system is that you are not trying to pull the teeth of the blind nut into the firewall as you are crushing the head of the bolt into the wood.

Good Luck!