This case has been watching me from above my desk, 90% complete, for the last 8 months. I sat down last night with a craft-knife, Dremel and sandpaper to finish it off:

The display windows/covers are 2mm thick transparent black acrylic, cut and shaped by hand and friction-fit into the milled chassis. These were cut to their rough size by scoring the acrylic and snapping against a hard edge, then refined with 180 and 320 grit sand paper to have a tight fit in each dimension. The corners were then rounded gradually to the right fillet by rotating each corner against a spinning sandpaper bit mounted in a Dremel:

Once the corners matched the slot they were filling, each cover was encouraged with a cloth-covered hammer to seat in its slot. Repeated tapping with a hard object didn't cause any of the covers to shatter when seating, though the shocks going through the case did cause a chip on one of the already mounted covers. Not too noticeable...just adds a bit of character:

If any of the covers do end up shattering, I have enough spare acrylic to fix a few of them anyway.

I originally ordered some 3mm tinted polycarbonate cut to shape in February to use for these display covers, however this ended up being too transparent to obscure unlit segments from view. 3mm was also too thick for this material, causing noticeable refraction when viewing the display off-axis.