That's an unusual battery set-up, our 2 certainly aren't on a raised platform like that, so may be peculiar to whoever installed them originally. I've just had another idea, not sure if it would work but ... behind the drivers and passenger seats as you've found the plastic 'grid' panels come off, in ours I removed these and put on some flat board instead which has given me quite a bit of room behind the drivers one to put a basket for dog leads etc on top of the raised step area, and behind the passenger seat gives extra room for our Romy to spread out (she's a heffalump!), so maybe there's an option to put the battery in a 'boxed' area on the floor behind the passenger seat? I've found that having the basket behind the driver's seat and some cushions behind the passenger seat (missing from my photo) doesn't stop comfortable sitting at all. Get yer tape measure out! Better still, make a model of the battery out of a cardboard box and see how it would work in reality.