Do wat he says with relay and connections!
On me Elmer I located second battery, behind the N/S headlamp moving the jack and chock behind the engine fuse box. all fits nice, but the jack has to be at its lowest to fit under bonnet, battery strap through the inner wings, just like the main battery, and if second battery too big for space, you can move the power steering tank to another location, or just add an extension piece and take it further back and out of the way.
Putting it there also means that you can connect direct to the alternator with a huge cable rather than run it round the engine bay, just use a relay to seperate batteries when engine off