I got a chance to work on my improved battery warmer.
Basically, it's just a tray of hardboard with spacer strips of plywood over the top of it. The heat tape runs between the spacer strips, and a layer of aluminum goes over the top of the whole thing.
Plug it in and the heat tape warms up the aluminum, which spreads out the heat to the batteries sitting on top of it.
I'll let you know more when I actually get it in my battery box!
