I have a crawlspace, and the main water shutoff valve is in very back so I can't get to it easily. I sawed a hole in the side of my crawlspace and printed this complicated thing. It's split in half so that it can be easily assembled from the outside of the wall, and so that the layer lines are aligned for better strength, and it prints on smaller beds. I printed in PETG, but I think ASA might be a better choice for long life and UV resistance. Print slowly to avoid warping on the bed.

This design was inspired by those waterproof caps on kayaks.

To assemble the screw-in cap, print out 7 hex dowels, slide them into the holes and the apply super glue all over the matching faces to create a solid bond. (see attached diagrams)

To assemble the inner flange with the threads, push each side in through the opening in your wall. Align the dovetails, and pull the assembled flange forward. Stuff some steel wool into the channel, and apply a liberal layer of caulk (acrylic if you want to paint it, otherwise silicone is fine). It should work for walls up to about 1 inch thick. The outer flange is thinner and is designed to be attached to the outside wall with more caulking, and it mechanically pulls the two halves together when the threaded cap is twisted in.