OK, I tried it with the thick "extra" spacer, and the two thin spacers that came with the new cassette.
I couldn't seat the lock ring...
So, I tried it with the two thin spacers that came with the new cassette, and an extra thin spacer from the old cassette, placed under the largest cog.
This worked..., except..., even when _really_ tightened down, I can wobble or tilt the cassette back and forth, ever so slightly on the spindle with my hands.
it doesn't seem like there should be any play at all...