On my car, I some friends and I welded in a chunk of metal that was the side of a trailer into the front of my car under where the radiator was. That gave me a nice flat place to set two batteries.
Here's a photo where you can see the "battery tray". It is the black metal surrounding and under the pink foam.
My batteries are too long to fit in there the way the one is shown. If you are using shorter batteries (group 24, etc) you can fit 4 batteries there as shown.
The way I originally started off with the batteries was to put two chunks of bedframe (free on large garbage pickup day) to span the spare tire well in back. The bedframe is sturdy and L-shaped to help hold the batteries in
place.
Notice in the second photo that there are still the caster wheels on the sections of bed frame. Bed frame is hard to weld, but it isn't that bad to drill through and bolt to the car frame.
If you remove the backseat, the metal of the car under it is still lumpy and not level. You could build a really basic wood shelf of some sort to level it out for batteries. I eventually cut out the back seat and welded in a box where the back seat and gas tank were. That keeps the batteries out of the way, with a low center of gravity, and keeps your trunk.
See, the back seat area is much better once you cut that middle chunk out.
After that, you can size it up right to weld in a battery box. Then I made a lid out of plywood with some carpet wrapped around it.
