My household deep freeze came with a retaining clip. Even though I don't actually use it, you can see how simple it is. Take the center screw out of the outlet center plate place the clip and screw into the faceplate. To take the picture, the clip is upside down. It should be underneath holding up the plug.
If you can't make one, perhaps you can search appliance repair parts websites and find one. (My freezer is a Frigidair.)