Wow, that is......silly!
Why...57mm?
What you could do is buy a better focuser, which would be my thoughts, and drop kick that one as far as you can!
The other thought is to have a small collar/reducer made that has a flange on it to stop it falling in that sits in the focuser draw tube and allows the MPCC to go inside it. I will draw a piccy of what I mean and post it shortly, gotta take the kids to school now.
Scrap that, it wont work!!
Can you have the adapter that you've got machined short enough to retain the locking screws and give you focus?
Other than that, it will be another focuser, unless someone else has seen this before.
