You can attach a dollar store bread pan to something and just fill it with the shell, they'll take what they need. I personally just throw it on the ground for them to find.
What do you mean by you only feed them once or twice a week? Is that when you top off their feeder?
If you really are giving them food once oe twice a week, you need to allow access to feed all day, every day.
First poster is right, protein is a major player on laying, so are other nutritional...