Here's the updated drawing of the adapter, now it has 2+6+3 = 11mm inside the focuser which is held by three set screws in the groove, it must be secure.
I've measured the outside diameter to be exactly 56mm (or slightly smaller), when I tried to measure in "inch", it gives a non-integer value.
And for the 57mm thread, it's 4mm long. No need to make a stop for the thread as we can see from the Borg's side, it actually has a stop there.
The original SCT adapter of the NGF-S focuser for reference:-
No need to make that long 50mm tube as found in the original SCT adapter for it's useless.