I am planning on doing some rally x events this year with my street car and would like to get some under body protection figured out.  I figured I could get a ruff idea of what shape to make it and where to mount it from pictures of them I have seen on the net and measuring my own car.  Just wondering if anyone has any advice on how thick I should use and what material etc.  Or if someone already has one they would like to sell at a fair price.

You don't need anything super strong for rallycross, like you would for rally.
My daily driver has a 1/4" LDPE skidplate, which I would have no concerns rallycrossing with that.  I don't rallycross that car for other reasons...
Because it's a plastic skidplate, it's lightweight and VERY easy to work with to make your own.  You can buy a large enough sheet, and cut it to shape.  LDPE is flexible enough that you can bend it to conform to the bottom of your subframe.  Unlike aluminum and HDPE which are not flexible, so you would probably have to leave it flat, and you'd lose some ground clearance.
Even though it's somewhat flexible, as long as you get a thick enough sheet, it will withstand anything that will happen in a rallycross.
From the LDPE wikipedia page:
<quote>it is quite flexible, and tough to the degree of being almost unbreakable</quote>

How did it turn out? Post a write up if you get some time so i can copy everything you did