Since you cant buy a battery that fits the battery holder. Are spacing the battery up to fit or are you reworking the holder to fit the battery?
That's easy, you take a small flat wood plank, cut some tabs and hang them in the carrier to fit against the battery.
Battery holder wood bottoms are available from some vendors. I believe Langs has them.
You can use a piece of plywood to easily make one.
There is no need to modify the battery carrier itself.
I believe Dave Sonoski makes them for Langs.
You can make your own from 3/4" thick wood.
Slips between the bracket legs and rests on the lower bar.
I put a spline on both ends so they will not spilt over time. Just a dab of glue at the center of the spine, though.
What Dan said.
I use an original Ford battery box. I had to make a few inside modifications to get it to work, but from the outside, it is perfect. I had a few bottoms made exactly like the originals, and they were gone in a week. I also had to put a thin piece of wood in the bottom of the box to bring the battery up to the top, so the battery cables wouldn't get cut. I also used an original NOS ground strap. The repros are fine, but they are not zinc plated as the original. Next I had to make two handles, and weld them to some thin pieces of angle iron so the lid would have something to rest on. No problems.