I take it the tray is stone resin? Yeah the trays i have seen are flat on the bottom, all you need to do is make sure the outer raised edges are level, the tray is formed with a fall (similar to modern baths.)
When i fitted mine, i built a frame out of 4x2 with a ply top. I made the frame top about an inch smaller than the tray (its a quadrant) i then used a no nails type adhesive to glue the tray, instructions said use cement.
As a finishing touch, as i had use the tray to mark my ply, so the profile was perfect, i then attatched a piece of white upvc to cover the base, was really smart. On my G/f's tray i used 5mm ply and tiled it with mosaic, looked even better than upvc!