Solid rigid PVC has a density of 1.4. It sinks in water. But foam it below a density of 1.0, & it floats. If you are buying PVC foam board at one of the big box stores - it comes in lots of sizes and thicknesses - look for boards with very fine interior cell structure... You should hardly be able to see it. That's the stuff that generally has both the lowest density, and the best screw retention for putting it together with fasteners. It should work well for you!