here's a pic of mine
it goes down and under the car stopping a few inches before the firewall. The idea is that is there to protect the radiator as much as possible. What's it made from? Stainless steel. Now before you go, "what the?! that must be so heavy". It's not that heavy and it's VERY strong. I went with stainless because all of the sheet metal shops round blacktown didn't have any Ali sheet at the time. The place I got this sheet, bent it up for me and every thing. If it is a little heavier than ali, that's an advantage in a tail heavy buggy.
Other things you can build the skid plates out of?
Fibreglass - works very well. make up some FB sheet. cut it half way through where you need to bend it, break it and then glass it back into the shape you need. Add some kevlar fabric while glassing it and it will never rip.
For non contact areas, rubber sheet or neoprene/urethane sheeting good to, to keep the gravel off the metal and stop the paint being worn off.
If you are broke and don't care about weight, mild steel is great. Hope that helps.
if you need more help as to designs and the like get this book...
http://www.amazon.com/Baja-Bugs-Buggies-VW-based-off-road/dp/0895861860/ref=pd_sim_b_4
It's the offroad bible for anyone taking early VWs offroad. Cheap and indispensible.