As said MI's but have fitted some where the carpenter has put batten on the joists with ply spanning the gap under the finish floor and bedded trays down with flexible tile adhesive.
Why would you want a tray on feet if you can get it on the floor, have confidence in your work.
Worst comes to worst, take a measurement below to where trap is and have to cut hole in the ceiling rather than rip out tray and row of tiles.