Hi Rob
I heard some people were using the early Pringles crisp tube covers (about 78mm diameter)
the early ones were opaque, and they worked quite well, but now the design has been changed to clear see through ones
. because the cap is a good size to use (fits over my 18-200mm Nikon lens) I cut a circle of tracing paper and attached to inside of cap.
Unfortunately I have not done any tests with Pringle tube expodisc!! When I get time i will post some test results.