1/2" fwd of the bend at the leading edge of the battery tray is where the front firewall is formed. You really don't need a box, just a wide crossmember bolted between the lower subframe rails. The battery will sit nicely ventilated on the new crossmember and be bounded by the triangular weldment for the steering rack in front, the firewall in back and the steering shaft on the left. A wood 2x4 spacer of custom size depending on what battery you use fits on the right.
To hook up, with a lot of fiddling, the stock +cable can be fed down to the new dropped position and there are lots of bolts down there to attach the stock ground cable. Now all you need is a hold-down strap. I used a thick rubber cargo strap with hook ends because of the restricted access once a battery is installed. You don't want to bolt a battery retainer on such that you'd need to climb under the car to access the battery, which will still need water etc. I don't recommend using a side post battery due to the same space problems.
I dropped mine 20 yrs ago and it's yet to cause any problems including 1000 mile road trips, open track events and the like. IMHO, the hardest part was actually cutting a big hole in the trunk floor! I made a sheet aluminum cover for the hole and made a tab on the aft end to fit in a slot and a dzus fastener on the other for easy access. Dzus can be opened with a coin- no tools required. As usual, a POCA Newsletter article showing all this, with photos, is in the downloadable Archives.