I use pieces cut out of the film that's used for garden ponds, called pond liner, made of butyl rubber. Found at the hardware store or garden center. Easy to cut.
Krist said.. Thanks guys , I sacrificed my child's gumboot for the cause worked a treat but maybe need a little thinner like the pond liner or M3 tape cheers
just found my next one... the base of a Woolies shopping. The price is right!