i am trying to fit a shower tray and going for mounting the the tray on 18mm ext ply sitting on 4" by 2" timber.i have just cut the ply to the strange shape of the tray and realise now that the two shower traps i have brought are not deep enough to pass through the ply onto the tray.how would you guys overcome this,is it possible to buy a waste fitting only that will couple up to a shallow bath trap?i did look in b and q but the only wastes they had were notched ones for basins,have not tried plumb centre etc yet,many thanks