Here is an article ( in German) that calls into question the attribution to Sophie Scholl. faktencheck page But regardless of the true source, I agree it is a powerful quote.
I like to use a traditional zafu cushion. After a few years, it lost some puffiness, so I refilled it with fresh buckwheat husks. I only tend to sit for 8-16 minutes, so it may take some getting used to if you sit longer.