If you are placing 2way mirrors on all 3 sections (I would suggest this since normal mirrors and most 2way mirrors have a different amount of reflection and different colors, it will cost more but look better)
Blacking out the back of the 2way mirrors on the side is easily done with vinyl (the kind used to make vehicle decals and wrapping cars with).
And I do believe @Goldjunge_Chriz had some good “tape” to make the mirror stay in place. Something similar to the sticky stuff they apply car windshields with I suppose?