Lots of them have no provision for screwing down, this is fine.
Adjust so that all feet are touching deflection free floor.
By the time you have siliconed the tray to the wall, then tiled and siliconed the tray to the tiles - trust me the tray is going nowhere. If you want you can run silicone/ stixall etc around the feet afterwards ( not necessary) - but don’t put anything flexible between the foot and the floor.
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Great its on 18mm marine ply screwed into solid osb flooring so should be fine Many thanks for taking the time to reply.