I would think that the easiest reflex shutter to design would be a drum shutter as found on the Exa model 1. Of course, due to shutter opening cutoff the longest lens that can be used is about 115mm if I remember correctly. Top speed on the Exa was limited to 1/150, also a limitation. However for a toy camera with reflex viewing and interchangeable lenses it could be a fun project. The Lomo Konstructor has a shutter somewhat like a drum shutter but has a very small, almost useless opening.