Heating Element (resistor) to maintain temperatures in the 80-150°F in an incubator
You can just use a chassis mount power resistor or similar type as a heater. You will need to size the heater- typically one might figure out how much power is required to maintain the highest temperature you need, at the lowest ambient temperature expected, and then (say) double that power. Here is an example resistor from this surplus dealer (but you can buy them from a distributor in pretty much any value you need): 



The voltage does not matter much, provided you have a supply available. Something like 12VDC would be safer for you to work with. 

You might try a small incandescent light bulb of known wattage in the enclosure when it is complete to get an ideal of what power is required.