You could always used a block of closed cell foam and cut it too fit.
That's a good idea - I'll have to think about it a bit.
The comments about "right of way" and sailboat vs powerboat raise some interesting questions.
Agreed. My understanding of right-of-way is dated and comes from what I remember in sailing navigation books back in the early 1970's. Based upon what I recall, craft under sail trumped all, whether they were encountering boats or ships (boats being anything small enough to be brought aboard a ship - except submarines, which are *always* called boats). Though in practical terms, a ship or large boat ("working craft") under power cannot necessarily maneuver to avoid a craft under sail, so the onus is on the sailor to back down and avoid the larger powered vessels. I always understood the smaller powered craft to be maneuverable enough to yield to sailboats, though I'm going to have to review that based upon your other comments.
Did not know about all of SF Bay being considered a "channel" - that was an eye-opener!
And, yes - I always understood a sailboat under power to be a powerboat. I would also expect that a kayak with a Mirage or Evolve drive to be legally considered a "powerboat" in a court of law.
All that said - I have never challenged a vessel of *any* size under power for right-of-way. I've always been in sailboats under 45 feet in length (mostly much smaller) and considered powerboats to be the top of the food chain because of the lethal damage they are capable of delivering. So when one obviously yields to me, I always take care to acknowledge that with a smile and friendly wave. And if they don't yield, I back off with plenty of space to spare.
You may even be able to (carefully) use expandable foam mixes to shape a perfect new plug.
That's a good idea, too.
The closed cell foam if cut larger and forced into place would take a significant amount of force to push out.
All good, thoughtful ideas - thanks! I'll have to cogitate on them for awhile.