On Mon, 25 Apr 2016 10:28:04 -0700, Don Y
wrote:

Generic credit card sized (business card sized?) PLAIN sheets of
plastic that are either very durable (unlikely to break when flexed)
*or* flexible.

Best guess I can come up with is a PVC gutter and a pair of shears?
(Hmmm... maybe a grade-school paper cutter might make easier
work of it?)
Buy a sheet of Lexan.,