Stack of quarters - they're approx 1.4 mm each
Quarters can also be used for tweak/droop in a pinch on 2wd's. Set one on each front tire evenly off-center to the front and lift the car from the center point with a screwdriver or hex wrench. If one quarter drops before the other that corner of the car is coming off the table first and thus has less droop.
One more tip: I found a used paper cutter from a school and use it as a tweak board. The surface is flat and the grid is perfectly square so it's easy to eyeball alignment.
GV2 Pro, XXX4G+, Blitz SCT, CRC GenXL