OK. This problem chronically drives me nuts.
I've tried many ways to support boards, furniture, etc... while spraying in my spray booth. All have proved to have disadvantages so the bottom of the object always is marred in one way or another. I usually spray Pre-Cat Lacquer, but have had the issue with other finishes.
1.) Painters Pyramids - I took some of these, weighted them down with buckshot/epoxy in the middle, and put sandpaper on the bottom of those, so they don't move. But invariably they make scratch marks on the bottom surface.
2.) Rockler Bench Cookies - Great to keep pieces from moving, but the rubber surface always leaves a round mark on the surface. I've seen it react to lacquer, and especially with varnish.
3.) Blocks of wood (seems the best, but invariably there are rectangular areas where overspray lands around them leaving rectangular marks).
4.) Lazy susan with paper towel on the top of it (seems in ways the best - certainly helps spraying), but also the issue with square marks from overspray, plus I have had the paper towel stick to a few of the surfaces.
5.) Rubber sanding mats - The worst. Totally keeps the project from moving, but totally sticks to the wood underneath at worst, and leaves a significant pattern at best.
So what do people use that actually works? I just haven't found it. I can spray a beautiful finish, but the bottom is always an issue.