Robby,
We are talking about different things. The o-ring you refer to is the same as the lower T-clamp o-ring. I did what you did on both my '00 and '03 with the large bevel washer and o-ring replacing the plastic cap, like the older bikes. What I did on my '07 was use the Scotts steel cap that came with the tower with a thinner o-ring underneath. It seals between the cap and the edge of the steering neck tube. The o-rings you have will be too thick for this. I'll measure one and post dimmensions tommorrow. Not critical though, the metric or closest standard equiv. will be fine.
Either method works.