I would suggest a ceramic heating element. The can run day and night and last for years. I would not use them with the typical dome type hoods people used but a metal lamp holder that screws into a metal conduit (both available in electrical area of hardware stores). I used to be confidant with domes and ceramic lamp holders that you can simply screw onto wood, but with bothmof these i notice the eat builds up gradually melting down the electric cable the dome socket this occures in the lamp socket or with the ceramic screw on type between the socket and surface.. the problem is lack of breathing surface to keep the cable cool.
The downside of your cage though is it will leak out a lot of heat, so you may want to change out any open air sides to solid wall.