I use onion skin method all the time on aluminium 0.2mm but you must ensure your bed is perfectly flat because it's very easy to break thru.
I also use lot of double sided tape for smaller sized material where can't clamp or screw and don't have excess waste material. You don't need to cover the whole material area with tape just in several places, depending on the type cutting your doing.

Washout DST works great with aluminium and brass. I trust it enough to have stuck large expensive sheets of brass down for engraving.