And here it is without the battery installed (just the metal jacket):
I bent some 1/8" angle and riveted it to the sides of the battery box, after cutting some lightening holes.
Three bolts to the firewall and this mini-project was DONE!
The only thing I have to do is put a bar on the top of the battery box to hold the battery in. Total cost for the nutplates and the AB4 aluminum bar is < $10.