These blanks are used where a cable or pipe will be fitted later. In this case the diaphragm will then be cut and the cable put through.

I am sorry I do not have any dimensions about the thicknes of the diaphragm.

Hint: If the hole is a little bit too narrow to fit the grommet, just use WD40 to get the grommet slippery. This wouldn't harm the grommet and the cables and fitting is much easier.