O-rings come in an almost infinite number of diameters and thicknesses. Try to gauge the gap, and look for a ring that is slightly bigger than the ID of the "skirt" on the headset, so it'll stay up there, and thick enough to sit above the outer corner of the crown. Otherwise the cut inner tube will be your best bet Also depending on how the crown recesses into the skirt of the headset (varies by brand) You might cut a donut from a sheet of plastic, and trap it under the crown, to support an O-ring above it.

I improvise headset seals as needed, but they're all different depending on the gap and recess between the cup and crown race.