I have put mine behind the drivers seat, its an odyssey drycell as I don't want a normal one leaking acid out in an accident.I bought a bracket for it off ebay too which is a good fit and made of alluminium(I sprayed it black).I then initially self tapped this to the bit of the floor that comes up at 90 degrees and would be behind your calves if sat in the back if you know what I mean.I wasn't happy with how secure this was though so swapped the self tappers for M6 nuts,bolts and washers and waxoyled it all on the outside.
TBH I wish I'd kept it where it in the place I 1st moved it to which was in the boot near the N/S suspension/light as it was much easier to access there but I was going a bit overboard with weight distribution.
Edited to correct.