someone told me that they offer shell and grit in tuna fish cans screwed to the wall in the coop...I am going to come up with something like it but use coffee cans(small plastic ones cut down to 1/2 size) that I can remove easily to clean...good luck
I just sliced them out with a hack saw.
Though I only just made them last fall, I unfortunately suffer from CRS, so I'll have to go out there with a tape measure tomorrow to check the sizes.