Virtually all control knobs sit on a 1/4" shaft - Amazon sell them - they do a nice Guitar pair for £2.93 post free - or a pack of 10 Black plastic ones for £2.53 post free.
All that should concern you is the length of the shaft you have protruding from the box - so the screw has something to bite into when you fix it on.
Alternatively rob one from a piece of redundant equipment you have lying around - some knobs just pull off and others have a tiny grub screw to hold them on.
Good luck...
Peccavi
April 2013
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