[quote name='BigRedX' timestamp='1461579437' post='3035746']
IIRC the screw length is fairly short - as the indent in the rack tray plays a large part in holding the unit in place.
[/quote]
Agreed,
Having measured with a cocktail stick as suggested, the hole goes right thorough to the other side of the case, suggesting there is also a hole in the main pcb.
That said, I think 10mm would be more than enough.
I'm assuming something like this will work:
https://www.orbitalfasteners.co.uk/en/products/m2x10-pan-pozi-machine-screw-mild-steel-bright-zinc-plated-din-7985?utm_medium=google_shopping&utm_source=google&utm_campaign=google_shopping&gclid=CjwKEAjwgPe4BRCB66GG8PO69QkSJAC4EhHhRMIeZwuFikz8HogWlMG0Tfe2h2Ug1eXpYTkIB0pn9xoCcWjw_wcB