You all are OVER THINKING IT, get two of the cheap ceramic light bulb holders and wire them in series tie them across the line and use two light bulbs of same wattage, (suggest 2ea 40 watt) 110volts will dissipate across each bulb... IF one blows both will go out so you need to know for sure one is working.
2nd option is to get 4 of the cheap "night lights," wire them in pairs in series. So "two sets" of 2 bulbs so if one set bulb burns out then 2nd set is still going. these bulbs are 7 watts each so that would be about 42 watts of heat inside a small box would be plenty most likely.
If you have OLD set of X-MAS lights you can do the same using them but those are usually only like 2 watt max bulbs.
NOTE: Series Wiring is Line (1st HOT Wire comes into Base#1 Line thru bulb and then the Neutral line of the base #1 out (into) Neutral line of Base #2, thru Bulb #2 and base #2 Line ties to the OTHER HOT line on the pump switch.)
Mark