What is the normal temp in your tortoise's pen right now?
If you only need a few degrees, try a regular incandescent lamp, a hot water bottle, or even a brick warmed in the oven and wrapped up.
If you need a bunch of heat, try a bank of lamps, heating pads, or even a hair dryer blowing gently into the pen- or combine techniques.
This is a good example of why I use multiple sources for my pens. Heat is provided by under-tank heaters (for 'belly heat' and general temp control), incandescent lamps, infra-red heat lamps, and a 'thermal mass' of stone and sand in the hot area. If one system fails, I can turn up another. I also have some spare heat pads and rocks sitting around for emergencies.
In case of a power outage during cold weather, I have a a few small propane heaters and a camping stove. One heater is for the reptile room, and the stove would start heating up bricks to be slipped into insulative sleeves and put in the hot areas.
If necessary, everyone goes to their hiding boxes so I only have to heat a small, insulated area.