I had a 300mmx300mm heat bed from a 3d printer that was slightly warped but still functional and did not want to chuck it so i added a few components, built a small box and built a plate warmer.
A old but working heat bed. - mine was 30amp
Power supply unit (psu) higher amp-age than the heat bed - mine was 12v, 360 watt and 30 amp- i might need replace it in the future.
relay of higher amps than the heat bed- mine was 50A + a relay harness to match relay
STC 1000 or similar - this is the controllable thermostat
Fuse and fuse holder and fuse optional although recommended.
Thermal paste and kapton tape to attach sensor to the bottom of bed.
Rocker switch - preferably one than can take the watts and amps of the heat bed but i never so used else between the 12v out and the stc 1000.
The appropriate size power cord, preferably with an earth wire.
Sheet of aluminium or steal to attatch to the heat bed to add studyness, i used about 2mm thickness.
wood,screws,insulating tape and all the usual junk to build a box
Step 1: The Circuit.
i followed the two diagrams for the relay and thermostat, although these are not exactly the right ones they was easy to follow to complete the project. Voltage may change for other heat box's depending on heat bed voltage, 12v,24v,120v or 240v
Ensure that a hole is cut under the box that lines up with the fan output of the transformer and that some form of corner feet are added to the base corners to allow ventilation.