Hey John, I used black cardstock from the newsagents. Sprayed it matt black & clear lacquer for waterproofing. Have included a selection of shaped masks to place over the hole & rotated to move the diff. spikes. When nec. I use both holes for focusing. The shaped masks & the 'dust plug' are simply 'blue tacked', (actually, I use 'UHU' which is yellow), to the underside of the main mask when done. Been using mine for about 3 yrs.
Works well...
L.