Why can't you specify a long distance between source and flat tested? it would minimise the effect, and negate the necessity of the lens altogether. Can't remember, but isn't it around a 20:1 ratio of flat diameter?
I still say it would be easier to shine a compact flourescent at the ceiling with a glare shield. It's 10 times better than a hanging light illuminating the flats as the reflection off your pan of water shows. The bands will be very evident.
Mark