Using a transistor as a heater
The power resistors will be the easiest.  You can get them with all sorts of resistances.  So 10W is 10V @ 1 amp (10 ohms) or 30V @ 0.33A, (~100 ohms).  I'm not sure how you are mounting them but I like the TO-220 pack that has a screw hole.  I've also used transistors as heaters.    Sometimes by themselves, but also in series with a power resistor.  For a single transistor as heater perhaps a FET is best.  I've used them at a fixed voltage and then pulsed the current.  (with opamp feed back setting the current)