Gasket material

I have sent away for some 1mm cork sheet from which to make an oil
resistant gasket for my raw water pump mounting bracket, but elsewhere
someone suggests (possibly joking) that a bit of serial packet
cardboard would do. The delivery hasn't turned up, and I am now eyeing
a rubber bar mat that I use as a computer mouse mat.

Mrs Nesbit came up with a complicated formula elsewhere for moulding
one too, and in the past I have used brown wrapping paper to make a
motorbike oil seal gasket (can you still get brown wrapping paper?)

Any other tried and tested ideas?