I installed two of these:
They're Mastervolt Slimline AGM batteries, and come in 150Ah or 200Ah capacities. They lay over nicely under the quarterberths, and I epoxied wood stops to the hull to hold them in place and mount the straps:
I have a house battery on starboard and an engine battery on port. Battery switches allow me to either split house/engine like that, to use either battery to run the whole boat individually, or to combine them if needed.