if you can make 2 tight nested boxes you don't need a helical mount.
you would remove the back off of 1 box ( which has your ground glass )
and strap on a film holder. the only thing you would need to do
is be accurate in your measurements, and make sure the ground glass is the same
distance from your lens or front of the box, as your film would be ..
have some sort of a strap or 2 that holds your film holder in place and take the photograph.
it can be as complicated or as easy as you want, personally, i would rather have something simple
it isn't worth the headaches and cash to make some sort of grand camera that may or may not be used very often.
have fun !