The biggest issue with these diy builds is that they need the mechanics to be inside the lens - which is not necessarily a bad thing, however it severely limits the lens choice for the system, and introduces additional cost (you basically need to buy a shutter with every lens). The scene has definitely improved over the years, there are a number of very interesting x-pan-like builds which have been made possible by advancements in 3d printing.

I'm looking forward to the day someone figures out how to modify a full frame shutter assembly (plenty and cheap on ebay) to work with medium format film.

For what it's worth virtually every medium format rangefinder uses a leaf shutter integrated in the lens. Many are simply fixed lens cameras (Fujifilm). Even kits with interchangeable lenses like the Bronica RF645 and Mamiya 6/7 integrated leaf shutters in the lenses.

To get a sense for what is required to have a focal-plane shutter on a medium format camera look at the Pentax 67. It's huge and heavy. Even with the mirror locked up there is a significant amount of inertia from the shutter transferred to the user requiring the use of a tripod in many settings.

A full frame focal plane shutter _cannot_ work for medium format sadly. It's be definition too small. I don't think you can use them partially in front of the focal plane, but I could be wrong.

Medium format focal plane shutters do exist, copal made them for Mamiya/Phase One for their AF, AFD, DF, DF+, and XF cameras. Some of which work for medium format film.

You don’t go out and buy a shutter separate. You can just buy a TLR lens that has the leaf shutter already integrated.