I took a different approach in my 2720SL, taking the lead from former long-time member Mike Laupp. I filled that cavity in with foam, which in my TM was on the right side of that compartment, and put a stiff piece of sheet metal over the entire cavity. I then used that Allied box Wayne referred to above, to perfectly fit 2 Trojan 6-volt batteries, and which spanned way beyond that cavity.
To secure them, I put a U-bolt through the metal plate they came with, then a small plywood scab, and then through the floor along each of the shorter sides of the box. On the underside of the TM, I put a metal plate over the bolts to act like a giant washer. Then through the loops of those U-bolts, along each of the shorter sides of the battery box, I looped in the hooks of a ratchet strap that goes over the box and lid.
Until earlier this year when I switched to lithium, that worked without issue for 15+ years and tens of thousands of miles with ~150lbs of lead acid batteries. I put the lithium in the same exact spot, but they're half the weight.
These are 2 really good threads on this topic:
https://www.trailmanorowners.com/for...ead.php?t=1696
https://www.trailmanorowners.com/for...ad.php?t=11180
Dave