Be aware that custom-machined polycarbonate, particularly using screw closures, may lead to serious problems with stress concentration and cracking. Do not allow your machine shop to use any kind of process coolant or lubricant other than water.
I recommend producing a set of prototype enclosures of the final intended design, using material from your expected production supplier, and torture-testing them for as long as possible to make sure that e.g. thermal cycling won't cause stress cracking at the screw threads.
I have had some luck with Plastite trilobular thread-forming screws in molded polycarbonate enclosures.
The calculator enclosures you mentioned are likely made from ABS or similar plastic, which is softer and less prone to stress concentration.