Both Sea to Summit and Duvalay do ones specifically to fit the California SE/Ocean downstairs bed with the cut out, if that helps. Possibly some others too.(E.g. Zone sleep etc) We have the VW Comfort mattress which we find very comfortable but we sleep head to the rear, and neither of us are especially tall so our hips are on the double thickness soft bit but if you are taller or sleep head to the front then some find it too firm.
If you need something sooner as a temporary measure and you are coming from camping, a foam mat or thermarest on top will do just fine until you find what you need.
I don't have the dimensions I am afraid.