I saw this idea in a magazine somewhere, so it's not all mine. But what I've been using are those thin cardboard inserts put into magazines to sell magazine subscriptions. An easy way to get a shape description on the cardboard is lightly coat the surface of say the face of the muffler and lay the cardboard on it and it will transfer to the cardboard. With an Exacto #11 just cutout the areas that doesn't have oil on it. Doestn't have to be perfect, but with a little practice you can get a pretty good facsimile. If you're not satisfied with the results, you should be able get it right the second try. Since those cardboard inserts are so plentiful, you should have an endless supply of material.