With the understanding that we're talking about less than 3mm, and an OD small enough to clear the stop tab, then NO Problem.

If buying a standard 10mm or comparable washer at the hardware, odds are that you'll need to file or grind the OD to fit. The bright side is that it doesn't have to be perfect, just small enough to clear.

‐----------

TRY THIS before doing anything else.

It's a hot summer, so treat yourself to an ice cream pop, or Popcycle. It doesn't matter what because what you really want is the wooden stick.

Now loosen and back off the RD mounting bolt. Break off 2 pieces of the stick and wedge them under the upper body. Tighten firm, but not tight, swing the RD forward to its correct position, and test shifting.

If this solves your problem, all that remains is making a permanent fix.

OTOH - if it doesn't work, at least you had a cool treat on a hot day.