After completion of the 3 previous steps, remove your project and prepare as desired (I sealed mine) ensure that ALL pieces are dry and complete before moving on to the building phase.

Building phase- take one back piece, and one front piece along with your ISO film viewers. If desired, you may cut off 3/5 inch of the outside of the viewers with an exacto knife, this allows for the glue to have a better “wood to wood” connection. Carefully, without touching the film, glue the viewer to the back side piece, lining up the viewing window. I used a nice quick dry crazy glue type. Repeat with the front piece. 

At this point, just to hold in place i snap on some clothes pins and allow to bond for about 15 mins. Probably doesnt need that long- but thats my method.