For the optional design with the lid coming down and plexi glass LEGO holder (red/blue version), use a router and small straight bit. Remember the sides must be mirrored. Route a line 4 ¼” long, ¼” from the edge and bottom. Route a line along the bottom piece ¼” from one side.
For the optional design with the lid opening up and plexi glass dividers (aqua/purple version). Route 1 or 2 lines across each side piece as desired, ¼” deep. (Suggested at 3 ¾” and 7 ½”) (not pictured)