Acebird,
Right on! I have built planes this way as well.
I don't know if you use this technique-Paint a coat or two of polyurethane on the cardboard first. Makes it more resilient, and easier to cut.
However, these planes are not quite as bulletproof as the coroplast jobs, although I agree that cardboard can be a little more sandable!
On the subject of fast and dirty, I once built a plane in 4 hours while working loading trucks- I took an engine and radio to work with me at Carter lumber, and my friend and I built it completely of scrap material-gutter downspout, blue foam, old scrap of sandpaper. Took it home and flew it. It was Really ugly- so much so that it refused to die! Finally got tired of it and took out the radio, and engine.

Jetts