other options include reptile enclosure heat pads and trace wires, some have tried ceramic heat bulbs that screw into edison screw light bulb fittings, but if mounted to close to a fridges thin inner shell wall can heat upto the point which melts the shell allowing ingress of moisture and microlife YUK!
ye olde worlde brew belts can even be used (shudder)
Lots of options, pick whichever suits you and your build, but imho the good ol tube heaters that never get too hot to touch are the #1 choice..
if considering using an incandescent light bulb, perhaps consider painting it matt black or sitting it inside a clay flowerpot to isolate the light.
also dont forget a fan to move the air about, it will be fairly inefficient to rely on convection currents alone to move the heat n cold from their respective radiators to the brew when needed, and everyone should be able to find an old desktop pc case with a fan inside and an old redundant dc phone charger or similar dc power brick 12v or less.