Here is a handy site that could be useful. It may seem a little silly, but if you need to make something of an odd shape this may be handy. Let say you wanted to make a funnel shape end for you exhaust, and didn't know how to create a blank. You can enter your sizes you need, and it will give you a shape that can be printed out, glue it on the material you want to use, and cut away and form it.
http://www.templatemaker.nl/