Max:
I searched for a long time, for washers to use as you described. I never found them, but I did come up with a substitute. On every MT 1015 assemble yourself parts sprue, is a very thin washer in the shape of the coupler box, with a hole already started in the center. Very thin, about 010. Ream the center hole out with any blade, nibble the outside into round and you have some quick, albeit dirty thin washers in almost any size inner and outer diameter you need. You can also slit one side, and then slide them over axles without having to pull the wheels.
Hope it helps.
Best,
Bill