OPTIMA YELLOW TOP. You can get one thats factory size or you can get a bigger one that will still fit.
IMO there is no other way to go, forget anything else. Armed forces and other organisations use these batteries in all kinds of heavy duty equipment where reliabilty is paramount, so if its good enough for them, you cant go wrong.
Fully sealed, awesome repeatable outputs, and very hard to kill, deep cycle so you can flatten it and not hurt it. In fact the easiest way to kill it is forgetting it on the charger and over charging it! These things charge very fast. There are imitation designs around that boast the same features, but optima has been around longer than most people realise.
obviously i have one of these babies!