Compact thermostat for VCO temperature stability
You could try something along these lines. Mechanical arrangement is up to you, but insulation will reduce maximum required power. Rt is a common 10K thermistor (eg. 0402 type) and R4 is an SMT power resistor. Pick R4 so that it is powerful enough to maintain the desired temperature at the minimum allowable ambient and supply voltage (preferably stick it in an environmental chamber to check it). 

It's basically a bridge where the thermistor temperature causes balance at Rt= 4.99K which is about 44.5 degrees C for popular types. R6 and C1 should stabilize the controller with a sensible amount of gain so it doesn't oscillate and cause audible noise (total proportional band is about 1/3 Kelvin) 







simulate this circuit &ndash; Schematic created using CircuitLab