I designed a 9mm blanks P.A.K. bullet box which holds 50 rounds.
The top slides on and is hold by grooves in 5 positions.
That way it locks in place to reveal 10 bullets at a time.
Easiest way to open it is by pushing in the middle of the lid, because that way, the “locking pins” disengage.
This is the version vor 9mm blanks P.A.K.
I also have a version for 9mm Para.