I also replaced the old tired 12V Marine battery with two 6V Interstate U2200s. I bought "blemished" ones for $45 each from the local Interstate distributor. They are perfectly good batteries that just have external blemishes but still carry the same warranty as the ones you buy in any store. As you can see I really had to squeeze to get them in there.
I also added a battery disconnect since I was changing everything out.
Finally, since everything is so tight and hard to get into, I added this battery charger port - an idea I borrowed from someone in the TT forum. I made a mated cable that I connect one end here and the other to my battery charger