You can thread nipples in and use conduit nuts to lock them in.
Nuts come in bag from Home Depot/Lowes, ect. Found in electrical section.
Tighten like this (Primitive Pete mod.)
You are just locking them in not pounding home.
Or you can use Plugs. (Thanks Swampb.)
Plugs are drilled and screws and double nuted for easy removal.
Also in electrical isle at Home Depot/Lowes.