I'd try and raise the body slightly off the cardboard when spraying. Its all too easy to spray slightly under the edge so that you get a line of clear where the body and the cardboard meet, which you then have to sand down. Not really an issue if you are only spraying the top or bottom, but if you are also doing the edges at the same time, then I would raise the body up slightly.
Obviously use something soft that won't mark or adhere to the finish. When doing the back, you can place supports in the neck pocket and pickup routs, but you can't use that method when doing the front. So maybe just a smaller bit of cardboard under the body is easiest.