Temps under 75F are not good.

Adding a small bulb, maybe 60 watt, should get you just where you need to be.

I would suggest using a lamp stand. They are $20 at both big pet stores. This keeps the fixture off the tank lid, reduces chances of a burn or fire, keeps the bulb from bouncing around which will make the bulb last longer. They can also be used to regulate how much heat goes into the tank. Raising it reduces the heat and lowering it raises the heat.

You can also put the bulb on a dimmer and dial it in where you need. This is good if you have a larger bulb but don't need is full power. Running a higher watt bulb at a lower power will make them last longer as well.

Something else to look into is a ceramic heat emitter (che). These are good because they do not put out light and last a long time. I've got some that had worked at full power 24/7 for over a year and a half. They still work I just didn't need them anymore when I got a designated snake room. They are a bit pricey compared to a bulb but they pay for themselves over time. If you go this route do yourself a favor and buy the wire cage fixture made for them to run it.