The biggest problem with converting your van into a sleeping machine is that you forfeit the ability to carry transmissions, beds, shopping trolleys, bits of lumber, and/or cattle, sometimes all at once. This is an easy removable sleeping platform. Simple!
Step 1: The Only Step!
Get four milk crates, and a piece of 18mm ply to fit the van. Remember - it must accommodate the inside diameter of the van door, or: make sure you cut it small enough to fit through the door!
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!