How do you lock AC cords into a standard duplex outlet so they cannot be unplugged accidentally?

You need a cover that is lockable, like the one from Home Depot. If you don't have a HD or Lowe's nearby, just show the web page to your local electrical supply store -- they'll have something like it.

There are also ones that have clear plastic covers, but not all are lockable and not all meet OSHA requirements.

You aren't likely to find anything on Ebay or elsewhere online.

Try one of these:

Replace the outlets with better ones that grip the plug prongs firmly.

Cut and drill a piece of plywood so that it will hold the plugs in the outlets,

and somehow fasten it so that it won't move.

talk to your electrician. not with a "standard" box. what you would use is "twist-lock" outlets. cords do not pull out as the connector has to be turned to come out and unlock. you would put the mating twist lock plug on the cord. they are used a lot for industrial applications when machinery is rarely disconected from the line.