This is the OP coming back again. Since it appears no one makes a waterproof controller (larger than 4.5A), my next idea is to mount the controller under the batteries in a waterproof box like the below one from Home Depot.
Home Depot 12" x 12" x 6" waterproof junction box.I'm in Alaska so I don't have the southwests' hot temperatures and I'd mount it under the battery boxes in the shade. I can easily seal where the wires go in and out of the box. How much heat does a controller for 2 - 300 watts put off - do I have to worry about it?
Bill