I didn't know where to buy one of these, so I printed one. (printed 2 copies actually, both being used) Apparently something like this is available at harbor freight. (not in Canada)

print 2 sides, screw on 1 or 2 sets of hinges, and enclose your extension cords, you can use cable ties on the ends to keep it shut, or a padlock or both. (like in photo)

there is some bridging near the end of the print, so be careful. Some warping also happened to me with ABS, but the end result was still usable.

Printed on a reprap-prusa-mendel-makergear with heated glass bed.