Above is a view of the o-ring seal on view port. The o-ring are viton(€16 each woops). 6mm cross section. The glass is a square in 12mm, normal type. I just got a glazier to cut it for me.
So the tube sits proud of the flange by about 3mm. This keeps the o-ring in place. I cut the clamping rings from 3mm aluminium. Drilled the top one tapped the underside one. It is split as you can see from the side view.
Oh and I pumped it down today, its tight! Yipee!