for headset spacers they are in compression , ID is all that matters, if you add a thin but wide spacer on top and bottom I expect It's fine ..

a Carbon tube spacer as sold is thicker wall tube making thousands of them they get the tubes made to Spec. then cut to length (on a machine that does only that).

Aluminum Cane Creek Interlok spacers go to a smooth outside for looks,
then their interlocking system on it's inside face slides closely over the steerer tube,
metal between is thinned, could just as well be the other way .

I bought three packets of Plastic Interlok spacers , and stacked them up to a 4" height.

[I have some left over] they are fine .. 9/8" ID 11/8" OD

But I'm using a Steel steerer tube .

George Hincappie may have finished Roubaix that day, with a steel steerer
even if a Carbon Bladed fork.

he instead crashed in a drainage ditch along side a Farm field on the way there .

He had an Aluminum steerer, and they, pro race mechanics, cut theirs short.

then again the several hundred year old cobbled Farm tracks are a National heritage site ..

... Unlike any thing in 'the States'.