remember that you have the potential of laying a
battery on its side quite possibly?
If you are looking at
battery sizes and eliminating any that are overheight, you will get a very small choice.
As an example, VW Transporter batteries to fit under a cab seat meant you were limited to around 100Ah - but if fitting on their side, you could get upto 160Ah ones to go in - and the Transporter bases were very small compared a fiat ducato type seat. base.
As an aside, I fitted a B2B to a Ducato and the owner had a pair of lead acid 100Ah batteries under the drivers seat. It was a very tight squeeze and they had to be low-height ones as they are next to each other but it was done.
I can't recall what batteries they were in that ducato but I used to fit the Leoch XR1750
Battery under the Transporter Cab seats -
https://www.alpha-batteries.co.uk/12v-xtreme-110ah-agm-leisure-battery-xr1750/ . These are good batteries, but maybe a bit dated and pricey compared to newer options, but in terms of low height batteries, they are a decent option.