Scrap Mechanic Password Session Locker (smpwd)
Locks your session under a password, so that even Steam friends are unable to join without also having the password.
Click 'Code' > 'Download ZIP' and extract the contents. Make sure you have Python installed, and run start.bat
By default, SM multiplayer sessions are checked for file hash integrity when joining. It's meant to be a security feature, but can be repurposed here to enforce that someone has to provide a password before they can join.