Connecting collar for Sharpes of  Dundee Keep Net.  Couple of different wall thicknesses (1mm and 2mm).  Adjusted to slide over the rivets which cannot be removed.  Overall dimensions are:

37mm Tall

20 mm internal hole (22mm Deep), 

12mm square hole (15mm Deep). 

Slot 5mm wide, 22mm Deep)

I hope it helps someone else :)

Added stl file for an extended collar to ensure a better fit around the mounting pole (Brian_Net_V3)