Monkey 44 --
As MatthewB points out Trojan makes great batteries, while one group 27 or 31 battery would fit in your compartment they have other sizes on their
website with more A-hr capacity, that you might look into to see if they would fit your compartment (GC12 and 921 group size).
Several other companies also make deep cycle AGM batteries that would meet your requirement for a single 12 VDC battery such as Concorde, Lifeline, Deka to name a few.
One other point to consider with AGM batteries is that while they are non-spillable in any orientation, they can be mounted in any orientation except upside down, so you might be able to fit two 12 VDC or two 6 VDC batteries in your compartment with them on their sides, if you need more A-hr capacity than a single battery can provide.