K1Max Chamber heater, using 'Handy Heater'

The cheap small wall plug-in warmer is sold with many brands (supposes 800W but reality 333W/230V)

NOTE! Mains power work, might kill you.

The heater unit comes with the all vital parts. You just need 1/2m 5 x wire and two wire mains power cable.

Discard all cover parts of the unit but keep the fan assembly.
You need to solder the longer cable between the display and main unit.
The power you'll get nicely out from heatbed controlboard's mains input (track from the mains switch).

Place the main unit on the bottom with 2 sided tape or screws through the side wall. You can turn unit on either from the control panel (there's 1h timer) or using remote control. With the remote you can change the fan speed as well.

After the chamber reach 36C the back fan turn's automatically on, turn that off to keep warmth in.

Ps. If you have space between the wall and heatbed +60mm this suites to the other brands as well.