You just graft the forks/downtube/BB from an old roadster onto one of these cheap Y-Frame MTBs. There's a guy who sells plans, but I didn't think I'd need em.
I'm currently putting together a chain tensioner so as to make the front DR functional; using a skate wheel and an aluminum bracket.
I've been thinking of refining this idea. Cheap aluminum Y-Frames are readily available, as are old aluminum roadsters (or even solid-frame MTBs.) By going to an aluminum frame and "boom", and greatly lightening the seat assembly and stem, I think I can bring one of these things in at about 30 pounds. That would put it in the ballpark for many commercial 'bents.