I've considered all those things. I talked to a guy at a local bike shop, he suggested I find an old steerer tube and cut a piece off to make the spacer. Even looked at mil spec washers but the correct OD would be hard to find.

If you look at my albums you'll see a pic of a "bike hidden stem extender" I think might work. Most reviews were good, a few thought it might come loose with a hard turn to the left.

I think the best way to mount it would be to split the difference between the steerer tube and the extender with the new stem with headset spacers below the bottom of the new stem and snug the stem bolts. This would ensure alignment for both.
Then tighten the extender bolt using the stem to hold onto to get it tight against the inner wall of the steerer tube. Maybe add some removable loctite?
Then loosen the stem bolts, add headset spacers to the top, and set the headset preload.
Then finally torque the stem bolts.
Ya, that will work, i'm sure of it (final last words lol)