I use a 12 volt gel cell battery on mine, however, the AE starter box does NOT come with the proper mounting hardwear to mount the gel cell inside the box.
You could probably fabricate something to mount the gel cell inside the box, but for as much as the box costs, it should have come with the proper hardwear to mount either flat packs or a gel cell. I opted to use the gel cell externally. I cut the alligator clips off of the lead wires and replaced them with two female connectors. The female connectors plug directly onto the male posts of the gel cell. It's a pain to have to carry the starter box and the battery seperately, but the gel cell's charge will outlast most flat packs.
Also, I'm not sure if the handle on the starter box would hold the extra weight of the gel cell if you were to mount it in the box. The handle seems weak and the gel cell is pretty heavy.
Ebay is a great place to look for a gel cell. I bought mine for like $15 and got a charger for $8.