@slomobile: Whichever way you go, you might consider using a 1 1/16" Forstner or anular bit (ideally spun by a cnc mill for accurate grid placement)on the face of the hardboard to positively locate each brass washer.
That's not a bad idea -- I was already moving to using a CNC machine to drill the holes, because having them wander aroudn even a little will ruin the whole look of the thing.
Whichever way you go, you might consider using a 1 1/16" Forstner or anular bit (ideally spun by a cnc mill for accurate grid placement)on the face of the hardboard to positively locate each brass washer. You can fine tune the depth to just barely capture the film/lens between washer and board eliminating the need for glue there. A small bit of glue in the recess can unobtrusively secure the washer if needed. The hole depth can also be used to adjust distance from LED to film/dissuser/lens.
The "filler disk" could asily be glue, or wood putty / paste (or something similar --- does not nned to be a washer / metal / "finished product"). Both harden and can be glued easily, and allow for clean up" before assembly. This can also be done to "mount" whatever lens you fit into the hole.
I suggest you make a filler disk with a center hole the same size as your brass washer. It doesn't matter what material you use, could be another very cheap washer. Glue your translucent disk to the filler disk. Glue your new assembly to the brass washer. Glue the final assembly to the surface of your project. If you select your glue positions carefully, the glue should be hidden when you are done.
@perl_geek: Two thoughts for the mass-production...
I'm currently exploring a variety of options. I must admit that the Fresnel lenses do look rather tasty -- but it depends on (a) how they fit in the washers and (b) if I can find a really low-cost source.
I have one on order at the moment so we can check point (a) -- depending on the result we can decide what oiur next move shoudl be.
Two thoughts for the mass-production; 3-D printer to create multiple copies at once, or a CNC machine to mill them. (Out of a large sheet, so lots created per loading.) At least the machine wouldn't get bored.
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