I would like to fit a Hoya R72 whose smallest available size of 46mm exceeds the 40.5mm thread of the rear element. Looks also that the dedicated rear filters for this lens are slim-frame types to avoid collision with the reflex mirror. I'm wondering if it's possible either to: 1. remove and cut down a 46mm glass to 40.5mm and mount into a 40.5mm frame? 2. make some sort of collar to fit neatly between the rear group and the protecting collar with a 46mm thread. Anyone been there?