lentildude, this idea won't work for you, but for everyone else...
Wouldn't this be the ideal time to get free batteries. Our president just sent tons of cars to the scrap yard with the "Cash for Clunkers" program. Every one of those cars had to be drivable (meaning good battery). They're all sitting on lots now waiting to be crushed!
Again, not the ideal battery for this application, but I imagine you could get dozens of them for next to nothing! Just call your local dealerships and ask where all the used batteries went and how you can get your hands on some.
lentildude, you mentioned putting them under the house. You'll probably be fine at your location, but I just wanted to note here for others reading this post that inside the house may be a better location because the batteries are most efficient at room temperatures. Please correct me if this is incorrect.
Also, my understanding is that the only reason a car battery isn't ideal for this application is that if you discharge it all the way, it lowers the batteries life expectancy. Couldn't you just build a simple circuit to cut off the battery from supplying power after it drops below 25% charge left? If you get them cheaply, you could just add extra batteries to make up for that 25% that you're no longer using. Maybe the other drawback is that automotive batteries will only last half as long.