Mini Modular Framing System - Corner Block and Rail

I wanted to make an enclosure for my desktop 3D scanner/turntable, so I don't have to turn the light off every time I want to scan something.

I managed to get some 3mm perspex covered in black vinyl, some at 400mm x 300mm and some at 300mm x 300mm. With these panels in hand, I thought maybe I can print some corner blocks and straight rails to join the panels together to form a black cover over my scanner/turntable. The key thing I have to achieve is to make sure no light is allowed into the box, so they have to join perfectly without any gaps.