a normal flat chassis just take 50% of the surface under the car, the one I saw just had holes for the wheel, it was folded in the middle to create a ground effect too(V-shaped)
HPI made a while back a prototype with a molded carbon chassis with the same kind of shape (the chassis were V-shaped and were totally closed (like a flying saucer!), had a diffuser too in the back, has something like 20 cells...but weren't able to make any record since the diff ball melted. very fast