Wood & Cement board water heater

[Scott H]

Sorry, you are thinking 1x4 or so?

 

[Hooligans7]

Yep, 1 X 4s (unless you use a string of holiday lights!) However, since your region can drop to single digits and below, you could use even wider boards for the frame, which can allow for placement of insulation in the bottom of the heater unit, for example, 1" rigid insulation. The bottom of the unit could be a "sandwich" of plywood as the bottom slice, then rigid insulation, then cement board on top, which would be adjacent to the light bulb(s). You could achieve R-6 or better with that arrangement, and never need to use anything larger than 40-watt bulbs.