I use felt wads in my brass sg cases. Plastic can't generally be used as they are too small
and don't seal the powder in place. As above, felt wad with card disc over the powder.
I usually make my own wads too, That way I get the diameter just right...

And yes you're right the brass is very much thinner than plastic cases. Also I do not crimp
the over-shot card into the top of my brass. I use a tiny amount of cyanoacrilate? adhesive.
I get folks giving me dirty looks when I tell them this BUT a small drip of glue dries solid breaks
away cleanly and leaves very little residue on the case. I have brass cases I have been using
for years. so this works well for me.