I designed a geocache container which fits to a post with a diameter of 60 mm.
The cap has a thread integrated in which the tube is screwed in. The is a little conus in the cap that should close it tight. Time will show, if it works and the logbook keeps dry.
In the meantime I placed it outside and it allready has been found for the first time (FTF) ;)
Mine printed well with LA at 0.2 without support or anything.
I added the FreeCad files if you like to modify it.