I tried 0.7 copper?0.7 brass?0.8brass? and 1.0 copper.
I got some plates from different dagpists all over the world when I was in trouble with electroplating.I found some use 0.5mm,some 1.27,some 1.5mm.
0.5mm copper is easy to be curved when you polish base cooper before eletroplating.My experiment is 1.0-1.5mm cooper is the best unless you can do the same nice word as Mike Robinson does.Of couse,brass is good to use too.