Keeps the o-ring clean and minimizes stretching.
I cut the zipper off a plastic sandwich bag and put it over the IP, after loosening the bolts and snipping off the old o-ring.
The new o-ring slides easily down to the bolts without getting roughed-up or grungy from the high-pressure head.
Then I cut the bag open to get access to the bolts.
The bolts can be individually unscrewed and the baggie can be easily slipped out from under the o-ring.
The o-ring snaps right into the groove. Then the bolt can be loosely replaced. I do one corner at a time.
Easy-peasy. The ring stays clean, tidy and un-scuffed from the head or the bolts.
This worked much better than I had imagined it would. Much better than running it over the bare HP head or stretching it over the delivery-valves after doing a corner. I didn't clean the head, except to swab the grunge out of the groove and the surfaces between the head and the IP shell.
The bag was typical... not the cheapest, but not a freezer bag either, ~4mil thick.