In theory a focal plane shutter is (or at least can be) quite simple. In practice it is probably the spring - and the means to tension and adjust it - that is the tricky bit.

The speed graphic shutter uses a really long curtain with several different sized slots in it. More sophisticated shutters use two separate curtains so you can vary the slot size - but then you are getting complicated and in need of machine shop facilities.

I always fancied a go at a really simple focal plane shutter for large format. I was thinking just two slots - one full frame (to allow focussing and slow exposures) and a narrower one for 'instantaneous' images - say around 1/125 s. I thought maybe of using an electric motor and plastic gearbox to spin the thing, too. These days it seems easier than playing with coiled springs, tensioning ratchets and gear mechanisms and removes the problem of the curtain slowing down as it traverses the slot (and loses spring tension).

Like most of my bright ideas... it is like number 149 on the list, or something. Not sure if I'll ever get there....

A speed graphic would make life a lot easier - but of course you may not be looking for easier ;-)