I have same prob with a Jewel tank (currently sitting empty in the alcove) I used corrogated cardboard before, but couldnt get it dead level. Dont really fancing altering the stang with it being no more than chipboard, and documents/instructions with tank say NOT to use polystyrene.
Just for reference, its definately my floor that isnt level (its a council house, nothing is level, nothing has a staights edge, even the walls have dips and rises)