The lithographic plate that the newspapers use to print from are .012 thick and make good checks.
Making checks and the thickness of material needed is dependent on the gas check shank dia on the cast boolit, and the sizing dia for setting. Thickness needs to be a bit more than sizing dia minus shank dia divided by two. With the Hornady GC at .018 thick I have sucessfully used .021 copper for high velocity loads and 2 thicknesses of .012 litho plate in aluminiun for up to about 1900fps. I did try a .012 and a .006 thickness of aluminium to replicate the .018 of the Hornady but there was not enough strength in the sidewalls to hold to the gc shank and they would come off. Some have resorted to glueing them on but I found that a thicker check of two x .012 was much better. There is no shank deformation with the thicker GC.

Von Gruff.