Just thought I'd share some links with you that I found while looking for something else.
Web Archive of Major General Tremorden Rederring
Another archived site with a template for the periscope
I first read about these in a reprint of a Donald Featherstone book. He called it a Tarr Periscope, named after Lionel Tarr.
Doing a search for that lead me to two more links:
From the Vintage Wargaming blog
The Geocities one is no good.
I did buy some business card size mirrors a few years ago and have some black foam core from another project. I hope to build one when I switch gears and work on some terrain.