Ideally, a bathtub tray will have silicone or rubberized grips on the bottom to prevent it from slipping. If yours doesn't have a grippy base, the most important thing is that it's plenty wide, so the edges sit securely on the sides of your tub. You can also cut a piece of wood in the exact width of the basin and attach it to the bottom of the tray. If you plan to keep the tray there permanently, you could use a waterproof adhesive and seal it to your tub. Just remember that you won't be able to slide it around or adjust it once it's attached.