Ideas on how to cast this shape

[Simso]

Hi folks,

So i am not a jeweller but a musical instrument repairer, lots of tools cross over.

Im stuck on how to do this one.

I need to make 4 brass replacement machine head (handles) for this guitar to allow tuning of the strings.

The existing ones were made from plastic and centre joined with glue, the centre has a 5mm square hole.

Im trying to be able to cast these as one piece if possible, just not to sure how to cast the shape with an internal square hole

3 photos,
guitar - missing machine head handles
One set of machine heads - these were glued around the square shaft
Two pieces together - my end casting goal

Thanks

Steve