Hanging something like a surge protector is a pain to get right. If you happen to have a photocopier sitting around, American Woodworker suggests you simply copy the back of the object for a simple template.
This works great for anything you need to hang with a hidden slots on the back that need careful measurement. If you don't happen to have a photocopier handy, a scanner (even a smartphone one) and a printer will do the trick as well. The photocopy template isn't useful for everything, but it certainly comes in handy for those trickier items.