If you have an OSH hardware store - go ask for "1/8" rubber diaphram in the sheet.
-- get yourself a 1'x'1 slice and that way you can make 2 of them. (one for you one for a freind, to or to messup on)
-- dont forget to take a manila folder with you and a pentel sharpie so you can make a pattern and then cut the rubber to match.
my
oil on cold nights stays at 80-90C in a small block ford. (326CI)