Presumably Ken this tray is going to sit directly on the floor if you are using a mortar bed to level it which means there will be no future need to get underneath it for maintenance. If this is the case it may pay you to cut a sheet of 25mm ply and use the mortar to sit this on the floor 100% level. Let the mortar go off and then sit the tray directly onto the ply after running a bead of gripfil adhesive around the underside of the tray edges where they touch the floor. This will mean the tray is stuck absolutely solid and completely level....And you only have to drop it in once! Click through on the project link above to go to our adhesives used at home project to see the gripfil and applicator gun.