It currently holds 4 Universal Battery AGMs.
The back box is 28x28x10, but there is a small (1/2") lip over the edge. It appears that the front box will be the size limiter.
There are many batteries (G27, G31) which are almost 13 inches long.... just too tight a fit. Several more are like 9.88 inches high, which is also a very tight fit. I'm looking at Deka G24 or perhaps even Optima. The Optima that fit are quite small, both physically and at 55Ah rating. dang.
It appears that the LifeLine G27 AGM battery is only 12" long. This would allow me to use a 100Ah C/20 battery, which would be good.
However, since I do not have a good charger or regulators, I may consider a flooded G24 this time, and eat the watering problems in the hard-to-access front box.
Perhaps I'll build a BMS on the flooded, so that when they die, I'll be ready for AGM.