These cases look pretty damn good! For the screw holes I would highly suggest making the holes large enough to accommodate standard threaded inserts, then once the case is casted, you can simply epoxy the inserts in place, it will for sure hold more securely than tapping directly into the resin (I wouldn't recommend that at all, unless you're only gonna use nylon screws or something, I can't imagine the resin holding the thread for very long if you use actual machine screws)