It's four inches thick! Wondered if anyone has tried it?
www.completeoutdoors.co.uk/Thermarest-MondoKing-Mattress-L
I currently have a Thermarest Basecamp I bought about two years ago, I'm looking into getting the Thermarest Dreamtime memory foam cover for it and cam across this mat 
They don't make a memory foam topper for this, probably because it would be HUGE