I am having the same problem. I think I'm going to get one of the grommets designed for metal panels, cut it in half, and epoxy one half to each side of the MDF panel. I'm using a rubber-coated cable clamp on the inside to prevent tension on the outlet box inside. (My cabinet's power comes from a heavy gauge line cord to an outlet box on the interior. A UPS is plugged into the outlet box, then everything inside -- computer, USB hubs, monitor, etc. -- is plugged into the UPS. Well, actually into a Bits Limited SmartStrip which is plugged into the UPS.)
-- Chris