Step 1: The Only Step!
You don't want it to cover the entire area, because that makes it hard to get in and out.
Then, place crates strategically and place board on top. I fill the crates with bottles of water (essential for my van which doubles as a high performance spa pool at times), tools, the jack, coolant, (as above) and other extraneous crap. This will weight them down a bit more too, as the entire arrangement does tend to leap into the rear view mirror when navigating potholes. However, once you pack the rest of your crap - surfboards and kiting gear and three square meters of sand - under the platform, they will move around less.
Step three - enjoy!