Cutting the firewall and fabricating a box would be possible. I was more concerned with drastically affecting the CG and needing even more weight in the nose to balance.
In doing so i would need to alter the tank floor and a few braces but nothing that cant be done.

What would be the best way to brace and recess / and attach the box into the firewall??

Im assuming if i do this the box will need to be removable so as to have access to the bolts from the motor to the mount yes>?