Slim Acrylic (laser cut) - top and bottom plates.
Measured by hand (calipers) and designed from scratch using QCAD.
Working on the top plate I encountered an issue with the design that I was attempting. There are a few feature on the top of the BBB that make a single tight fitting top plate not practical, because it wouldn't be sturdy or provide significant protection. So for now I have put a pause on the top plate. I may come back to working on it some time in future, albeit with a compromised goal.
Thinking about adding 'stacking block' nubs and holes so blocks can be attached. This might also help for holding the 'stacking block' compatible rod for an axle. Since I'm sticking to laser cut this might not be practical so I'll look to other features to make it convenient to connect the plates to other components, like for building a car body around it etc.
Currently: Port Holes are nice and tightly now.
Top Plate Connection Hardware