Rayyy

I've got a 1600x800 stone resin shower tray from BnQ. I've made a 4x2 timber frame + 18mm marine ply to raise it off the floor( didn't like how the legs that came with it were rocking the tray) My question is about how to go about putting the tray down. Manual is very vague and only shows pictures of either using the flimsy legs or just putting the tray down with no legs and just adding silicone to the sides. My ply base is pretty sturdy and the tray sits plumb on it. Level is almost perfect but to get it as perfect as possible I might need about 1-2 mm raise on one side. Seems like nowadays most people use tile adhesive/silicone/grab adhesive/foam etc. Im kinda leaning towards just bedding it in tile adhesive. Or would using silicone or other similar products be easier since the tray is quite heavy and setting it down in tile adhesive will be a lot of work? Would appreciate some advice sine I've been overthinking about this particular issue and I haven't been able to go forward with finally setting the tray down.