The kids are somewhat enamored of this particular species of monkey:
But that's the construction. The kids asked for weeks for a periscope. So...
I used scrap wood (which is a recurring theme of my projects) and made the sides out of 1x4, the tops and bottoms out of 1/4 inch plywood. The mirrors were from Michael's. I used the Incra protractor (love that little toy) to lay out the 45 degree angle, and it just so happened that the mirrors were exactly as wide as two 2x4s laid side-by-side. I used E6000 glue from Katie's stash to set the mirrors. Finishing nails were used in the final assembly, and retrospectively I should have glued it as well. All in all, it came out well.
Fit and finish wasn't too bad.
And optically it seems to work ok:
The true test is the "use and abuse" test. The kids still play with it frequently, and so I guess we can call this a success.