Mount the engine mount to the firewall directly (outside the plane) to seat the blind nuts. Take the mount off the firewall. In mine I didn't glue the firewall in. Use your mount as a guide to pre-drill the holes in the front of the bat. Now, take two long straight pushrods and stick them through two of holes in the front of the bat as guides, slip the firewall on to these through the radio access hole, tip the bat up and let the firewall slide to the front inside of the bat. Start screws in the other two holes. Remove the "guides" and install the other two screws. The cool part of not gluing it in is that you can now reuse the firewall over and over.

I did the original flat bat as described above, but I didn't use wood, screws and blind nuts. My firewall was made from POLY kitchen cutting board (wall mart) and the mount was secured with #6 x 3/4" self tapping screws into the cutting board.