Sewing a car seat cover/protector?

I am hoping to make a car seat cover, or three, for our trip away.

Anyone have any suggestions?

I was just going to make something simple, something which looked a little like a fitted sheet, with gaps for the straps obviously. What do think? Does that sound right? Or do I really need to use a pattern?

Oh, and what kind of fabric? I just want something that keeps food crumbs from the inside of the car seat as its not the easiest to get off... but i also wondered... if she got sick, or had a leak... it'd be good if it was thicker and possibley waterproof...

Give me some suggestions, I'm not sure? The only ones i have found at stores are $50! Not paying that much!

Not sure if this is what you were after - when DD was toilet training DH rigged up an elaborate affair to protect his beloved car interior. So he bought plastic tablecloth material (can be cut to size required) from Clark Rubber/Spotlight etc and put this over the seat - holes cut for belt etc, then placed a towel for added absorbancy between the plastic and the car seat (was safe as seat was as secure as without the plastic). He wasn't fussed about the actual baby seat as the cover comes off quite easily and dries overnight. HTH