Wrightpt1 I have not add another polarized layer, because i replace the entire screen of the iphone 5, and also replace the glass of the ipad 4, the old polarized films were removed.
I think it may work adding another polarized layer, but i haven't tryed it yet. The film from ebay look very good. You can cut the film for smaller screens after finding the "good" angle. Also you don't need to glue the film to screen to test it, just place a litle duct tape in the edge of the film. To find the best angle you need to try it, because each screen have diferente angles, that's way it's importante to order a film bigger than the size of the screen you want to use it.
For bigger screens, you can find the "good" angle with a smaller film. After you know the "good" angle, you can order a new film with the right angle and size.
Keep us update! Hope it work for you. Also don't give up if it don't work the first hours/days. The eyes (at least my eyes), need to rest a few day/weeks after using "bad" screens.
Good luck!