I built a belly pan after reading Daox's thread over at MO
(
03 Matrix Mpg Build Thread - Toyota Matrix Owners Forum).
My first one (pictured later in the thread) got damaged by a tire in the road. I used it as a guide for a new one. I still have it in the garage if you are interested in it, or I can make a CAD drawing of what I did.
2006 Matrix, 1.8L 1ZZFE 5 speed manual!