Just a large comfortable knob to place over the filament release bolt on the extruder.
This is a perfect slip fit for my machine but I wanted a nice fit and since not all screws are identical and not all material shrinks at the same rate the center hole was given no additional tolerance so it will need to be reamed outfor final fit. (Or print at plus 1% or 2%).
The bottom was intentionally left round rather than hex shaped to make sure it never loosened the nut.
Center hole might need to be reamed for final fit depending on material shrinkage.