This idea doesn't require much more space to implement. However it would be much more difficult to make, yet virtually impossible to get the pin to lift back out of the trap.

Anyone familiar with how a spring loaded pen cap works should easily know where the idea came from.

The concept is pretty simple. The pin goes down, rotates a little bit, then simply won't go back up. Just like a pen.
The hardest part to make would be the cut inside the chamber.