Right - so, here is Daniel’s and my periscope.
Actually, we figured it was much easier to just pull the cube’s edges a bit up and down, as you can tell by the edges visible in the screenshot (edit mode). So, our scope is really just the cube, but slightly skewed. Was that cheating?
Also, we moved the ORIGIN of the periscope to the middle between the two lowermost vertices and then set it to (0, 0, 0), which I think is a nicer solution.
Regards
Carl