Small waterproof / weatherproof solar controller - FOUND!

There are all kinds of waterproof controllers on eBay for less than $25 in the 10-20 amp range. Mostly used for outdoor lights AFAIK.

You could get two or three and put the controllers in parallel on the batteries to add their amps, and have as big of an array as you can manage :)

Some say MPPT on them, but ignore that if they do (they are PWM) Also beware of voltage input specs. Some are as low as 21V limit Voc. (Many 12v panels have a Voc of 22.x volts)