UPDATE!!!---New tutorial "How to balance acrylic mirrors". www.instructables.com/id/How-to-mount-and-balance-mirrors-for-spirograph-pr/---Acrylic mirror is very light, so double sided sticky foam tape will do job. Piece 1/2 x 1/2 is working good.You ...
It's a good habit to keep dust away from optical staff, so our device needs hermetic enclosure.
I had Hammond 7x4x2 box lying around, so I put it in business.
Since we determined optic configuration and beam path we can mark and cut out window. Then get square piece of transparent acrylic and glue it to its place. Next drill one more hole for power jack, glue it, connect to board and we are done.