A groove is cut on the backside of the cap until the center section falls out.
The cut is widened at this depth to accomidate a 55mm filter with the glass removed.
The male thereads are cut off the filter and the ring epoxied into the recess previously turned.

The hole can be cut with a coping saw or a Dremel if you don't have a lathe.
The white caps seem to work a little better for this application, though the black ones might look better.