Want to embellish your periscope project?
Consider adding a salvaged automobile side view mirror's adjusting mechanism to the top periscope mirror. It can be the electrical or mechanical type, although the older mechanical types are harder to find. With that adjustable mirror you'll be able to see up & down (to some degree) as well as left and right.
The upper parts can be installed in a right angle electrical or plumbing fixture, plastic or metal- -just as long as it is water tight.
Anther far-out idea is to make your periscope into a "Camera Obscura," The adults as well as the kids will like that. I remember the one in San Francisco at Cliff House from years ago. Don't know if it is still there.
And Rob, after you get through restoring your collection of Airstreams & Tow Vehicles, you might want to start the periscope w/ camera obscura project. Zoe and the boys will love it.