I can't see a 25 watt bulb throwing much heat and I wonder if any will get through the tubs sidewalls when the temps in the room get down below 72ish.

Those cheap lamps have plastic instead of ceramic bases to screw the bulbs into and are a risk if left on for extended periods of times, although admittedly with a 25watt bulb probably not much of a risk.

If my rack had to endure cold temps every year I'd have 11x12 inch piece on the hot end and a 3inch strip for the cool end.
I use a space heater and keep the entire room about 78.