The old machining and watchmaking books suggest soldering thin parts to
a custom fixture. If you tin the part, wipe off the solder and clamp it
in place while heating it the error due to solder thickness should be
small and repeatable.

I cut a shallow recess in a soft collet or the top jaws of a
Microcentric chuck when I need to turn a thin washer that can't be
surface-ground. Sometimes the part also needs clamping pressure from
the tailstock

I've made an aluminum hydraulic pump rotor shim 0.008" thick by etching
a thicker machined part with drain cleaner.

jw