Heating pads might do the job in that small of a space. You don't want the water temp to get much above 70 degrees, but the air temp for the tomato's should be in the upper 80's. I might also suggest using a small room oil filled radiant heater like this:
Electric Oil-Filled Radiant Portable Heater. However with a space that small it might be hard not to over heat the space. But they do make some with a digital thermostat built in, though those are more expensive. Also you can always try and increase the air ventilation to compensate by bring in cool air, and another possibility is setting the heater on low and using a timer to turn it on and off. You can also use a heat lamp, but it will over heat the space as well unless you regulate it's on/off times. Also with a heat lamp you would need to be very careful in where you put and how you set it up, or you could cause a fire.