I bought a couple of 3" PVC elbows ($2.39 ea., http://www.homedepot.com/h_d1/N-5yc1v/R-100346018/h_d2/ProductDisplay?langId=-1&storeId=10051&catalogId=10053&superSkuId=202905035) and a 2-ft. PVC pipe ($5.44, http://www.homedepot.com/h_d1/N-5yc1v/R-202018043/h_d2/ProductDisplay?langId=-1&storeId=10051&catalogId=10053)
I had already a couple of 3" square mirrors from Michaels (sorry, no link here).
Then I cut two slots in each PVC elbow using Dremel Multipurpose Bit 561 (good for plastic and wood).
The rest was easy: adjust the mirrors so they are at 45 degrees and make sure you see through the periscope. Then fix the mirrors using a glue gun.
The periscope is ready!