I have a thin gap between a disk shaped component (7.5"OD, 0.5" thick, lets call it the "mount") and a long pipe serving as a vacuum chamber. I'd like to fill this gap with something to reduce the conduction between the two volumes on either side of the mount. The mount already has a sizeable hole in its center, so we're not talking about achieving an airtight seal here, just some rough baffling. The pipe it sits in is 7.8+/-0.1", where the rough tolerancing comes from weld beads and other surface variations. I pull 5e-9 torr so the material need not be quite UHV but still pretty good. The mount must be easily installed and removed. If it weren't in vacuum, I'd use weather stripping, but that's obviously out of the question. Even if the plastic/rubber used in weather stripping was as good as SS316 for out-gassing, the foam structure would constitute a massive virtual leak. The other obvious choice would be to build a ring-shaped piece out of metal to block the gap, but the rough tolerancing of the pipe makes this challenging. Epoxy might work, but its not removable. Any clever ideas?