Nimble Breech Wedge

This is a simple wedge that goes inside the Nimble Breech, in case you f*** up your breech and it won't stay close anymore.

**Side effect**: after using it for a couple of days, the breech feels firmer and will now stay in place without it. I continue to use it anyway because I think it helps on more elastic/flexible filaments.

**Improvements**: I would like to chanfer/round the sides a little, and I also like to publish the `.stp` files for stuff I make, but I am a dumbass and forgot to s