There is this other mod I use called Passage Intersections + More. They have enclosed passages that have a thin floor added to them. I actually like your Passage Merge Block even more than the tunnel, I just kinda wish it had the floor.

I can add a second one with a floor varient, rather give people the choice. I most likely make the floor the same as catwalks

Can you make a version that can extend and reverse like a Piston / Hangardoor?

Sadly no, space engineers block definitions only permit one definition per block. So i can either tell the game its a merge block or a piston but not both. Its really annoying how the have scripted things.

can you add one then that accts as a piston?