It is also possible to shim up the o-ring, and perhaps use a harder one. A thin strip of paper, cut to fit at the bottom of the o-ring groove will also tughten up the front shroud support. If one revolution/thickness doesn't do it, cut the strip longer and go around twice...
Or run like the springer breech seals and wind some detal floss in there. suggest gore Glidefloss as it is thin trflon tape.