One more vote for a 800mm by 490mm piece of 10mm ply. Finally got round to getting a piece of premium plywood cut at the DIY store today. I have both a space saver and a full size I can use as a spare and the socket on the floor. I'll start with the space saver I think. The floor gives a little but not enough to cause a problem with the kind of loads I carry. I may fit a few battens to the underside but it's actually OK.
Thanks be to different1936.