I'm after a couple of bits of MDF to fit speakers to my car - 2 rings approx
6 inch OD and 5 1/4 inch ID, and if there's some easy way of doing it, and
ideally the rings would be tapered, so that I can have the speakers pointing
upwards out of the bottom of the doors, and a couple of bits that are
straight on 3 sides and wavy on one of the sides, probably in around 9mm
MDF.

For some idea of how the rings would look, here's an ebay listing for some
in the US.
http://cgi.ebay.co.uk/6-5-3-4-MDF-Di...d=p1638. m118

Obviously the almost rectangular bits would be fairly simple to cut with a
coping saw, but I'm a bit stumped about how I'd get a taper with the rings.
Are there any places that'll supply the wood and cut it? I very rarely do
any kind of woodwork, so buying a lot of tools isn't going to happen,
although there is a lathe in the shed that I could use, although I can't see
any way of using a lathe to get a taper...