MATERIAL TO MAKE GAS CHECK SHANKED:
Ultimately based on YOUR bullet shank diameter vs final sized bullet.
The following are the general required thicknesses:
.22 cal- .012
.30 cal- .014
.35 cal- .012
.44 cal-.014
.45 cal- .014-.017 depending on whether handgun or rifle caliber.

“AMERIMAX” brand flashing is a good product and offers various thicknesses and is available in most U.S. home reno stores. In Canada,
the brands available are usually restricted to .012 thou however AMERIMAX is readily available online.
A 4-inch x 50-foot roll will make @ 10,000 .30 caliber checks.
You can make the checks out of any material that is not hardened which is the desired thickness.
You have the option of “sandwiching” material as well to get the desired thickness, for example, .012 flashing material plus .004 beverage can material will produce a .016 thick check.

MATERIAL TO MAKE PLAIN BASED:
Easy to source “beverage” can material, it is @ .004 thick and will form to the bullet base without displacing lead when sized.