I think you can get that universal gasket stuff a bit like cork or cardboard. You just mark up what you want and cut it out. Im sure ive seen that used before on a TV proggy.
EDIT: here we go, something like this:
http://www.frost.co.uk/item_Detail.a...vent%20(500ml)
Look a bit futher down the page S170 1/32" Oil Paper (0.5 x 1m) / S174 1/16" Rubber Cork (0.5 x 1m) In Stock amoung others on the page.