I have the factory mattress in my 06  QS10 and wish I had a bit more padding.  When I roll over my shoulder and hips feel the foam bottoming out.  Due to its age and the fabric showing some wear I want to just replace it.  I think my current mattress is about 3 inches thick.  Just off the cuff I am thinking about the new one being about 5 inches thick overall.  I have interlocking padded foam that is 1 inch thick and brand new.  I am thinking about gluing that to the bed and getting a 4 inch mattress on top of it.  Id like to hear what types of foam you have used and been pleased with as well as if you went thicker.