[MetroMPG]

An air dam is easier; a belly pan is better. Look at the most efficient vehicles - they go for smooth underbodies.

Someone here used metal screen (like window screen) to create their undertray, then painted it - the paint fills the holes in the screen, making it a smooth "air tight" surface. I thought that was pretty clever.

Someone else here has a Saturn with framing underneath, and the panels were attached to it. Sorry I can't remember which one!