I’m having a bout of insomnia so you’re in luck...
I looked at this for lowered bases before and decided it wasn’t worth doing because of height restrictions. With a stock seat base, you’d have better luck. Ignoring the thickness of floor padding and measuring raw bases on my garage floor, I get roughly 11 1/2 - 14” wide x 14” long x 7 - 8” high. I’m giving a range on some of these dimensions because there are various obstructions so you can’t use the full dimensions in some spots. The floor padding (which could be cut away) is about 1/2 - 3/4” and it looks like you might get an additional 1 - 2” of height inbetween the seat mountings on the seats themselves.
Height is your biggest restriction so I’m ignoring any meaningful FLA batteries and assuming you’d mount AGMs on their sides. I’m ignoring lithium and other ‘exotic’ batteries here.
I’d say two popular standard options should fit. Either a group 31 battery (12.9" 6.75" 9.27" LxWxH) or two GC2 golf cart batteries (10.38" 7.13" 10.88" LxWxH).
So it comes down to the highest AH batteries you can find around these sizes. A typical 12 volt group 31 battery is around 100 AH and you only need one. A typical 6 volt AGM golf cart battery is around 200-220 AH and you’d need two. (One under each seat)