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Very Nice,great work

Thats because the epoxy used to make the knot base extends far up into the hair knot,not alot you can do accept open the hole more.
Some will say to use an O-Ring,your call.

Can you get it down to about 54mm,if you bowl lather that might work out,may be floppy tho.

Plaster of paris works great to build up the inside.

From the top cap to the tips of the knot,50mm should serve you well.

Set the knot at 50 mm.