Also if its not possible to install one of them trays due to height increasement then you could install 9mm backer boards on the floor to gain a bit of height up to where you want the falls to start, then form your own falls towards the drain using this product from bal .......
BAL Quickset Cement
Specially formulated cement for the preparation of rapid-hardening low shrinkage mortars to be used as screeds or renderings in interior, exterior and underwater locations. BAL QUICKSET CEMENT is sulphate resistant and ideal for use in wet areas including showers and swimming pools. Screeds are ready to receive foot traffic after 3 hours and ceramic tiles may be fixed after 24 hours. After 24 hours, the strength of BAL QUICKSET CEMENT screeds exceeds the level normally attained by conventional cement:sand screeds after 28 days.
..........dave..