Players don't need to sign up in advance. Just show up. First come first
serve. Sign up on a sheet and form your own matches. Play for 40 minutes.
First 24 play first. Then others sign up for the next session. After you
Play you can sign up again.

A variation on this can be players must sign up and pay in advance for
24 at 6:00 and 24 at 6:40. After 6:40, players that don't sign up in 
advance can play.