Can someone please advise me on what's the best way to lay a low level stone tray (1400 x 800) on an un-level concrete floor? it's low by about 6mm on three corners. I've dug a channel to take the trap and waste. I've already tried with a bed of sand and cement but couldn't get it completely level so took it up and scrapped the sand/cement. Any advice would be very gratefully received, this is giving me insomnia!