I would have to agree with Morton on MDF, the big book of games seems to push it however so I will keep my mouth shut on that subject.... Carrying anything on your cars roof seems like a really bad idea, much less full sheets of wood, maybe borrow a trailer or pickup next time and use this as a learning kind of thing. As for the stain, this would require some planning before the fact, #1, Keep the construction very strait and use a very minimum of the appropriate wood filler (utilizing the sawdust from the wood you are using) #2, don't scratch it all up during construction (work on clean plastic, or rags) #3, test your finish on paint stick samples and decide what you want before you just go splashing it on. (*Hint for a really light finish you can clear it first, and that brings up the grain, than float your finish over it lightly and clear again)