Hey Roger,

When I replaced the foam gaskets on the utility outlets on my van I used a
rubberized foam material I found at an arts and crafts store.  They
carried 10x12 sheets of the stuff in different thicknesses.  I cut them to
size with an Exacto knife and then used a corer to make a hole for the
screw flange things.  Since this foam material was fairly stiff I made the
holes just a bit smaller than the flanges.  This way when I forced the
gasket over the flanges it made a nice tight fit.  It seems to me that the
stiffness of the gasket material will largely determine how big of a hole
you need for the screws.  If it is really stiff the hole size needs to be
close to the size of the flange, if the material is highly flexible (like
the gaskets BD carries) then the hole could be much smaller.

Hope this helps
jake