My bad! I must have been out of my mind when I posted that. Walmart has a tender for $15 but it isn't waterproof and thus you can't install the box under the hood. It's workable, but this is the one I bought:
Amazon.com: Battery Tender 022-0150-DL-WH Waterproof 800 Battery Charger: Automotive
I'll go back and edit my previous post.