I've found that BT-55 fits perfectly around my hexagon BT.
So if I cut a lengthof BT-55 and then paint it silver I can put it partly over the end of the Hex BT for the ferrul.
The rubber can then be a bit of crafted balsa, sanded to look worn. I can then put an 18mm hole in it and insert a friction fit motor mount then painted pink The whole eraser assembly can then be inserted into the over hanging 'ferrul' section and glued.
Does that sound OK, can anyone foresee any gotcha's?