There are some vendors which sell wrenches specifically for ER-20 tool changes — I bought an ER-11 wrench from Maritool, so feel comfortable recommending them:
(confirm that you need a 30mm between flats)
another (Made in U.S.A. last I checked) option are the thin wrenches from McMaster-Carr as described at:
specifically see:
For something else entirely I went for a wildly different option, because the spindles weren’t available then, and I was doing a lot of test cuts for various reasons:
but really, if one optimizes the tool ordering, an ER-style collet tool change is a pretty quick, easy thing once one gets accustomed to it.