First try to retighten the 4th screw to try to reseat the blind nut. you may have to pull on the bolt a little to get it started. This should work as long as the teeth on the blind nut are not bent over. If it works tighten it down well and then you should be able to remove the screw again. (remember what one it is so you don't push it back out when you replace the screw. If this does not work you may have to make a hatch to access the rear of the firewall. I am not sure about the platic fuse but with balsa you would go thru the bottem just behind the firewall. If you wish to beef it up you will most likely have to double up your firewall and this would mean accessing from the rear anyway.
Good luck!