I installed the Hart & Cooley rain cap when the roof was built, 
about 12 years ago. To my surprise for several years thereafter
there was water on the floor of the closet below the rain cap.
To protect the floor, I placed a 5 gallon bucket at the site of
the drip which in the course of time became so slow that any water
in the bucket evaporated. I never had to empty the bucket. For
reasons I don't understand, the bucket is now dry. Maybe I mistakenly
blamed the Rain Cap to be minimally incompetent. Maybe the leak was
around the flange and has sealed itself with local tar warmed by the
sun.

Our options now seem to me as follow:

a) hypothetically one could remove the Rain Cap from the short segment
of 7"-9" Hart & Cooley chimney pipe that protrudes through the roof
and replace the Rain Cap with an imperforate cap. Since I can imagine
no other use for such a cap, I doubt that it is available. Hypothetically
one could purchase the shortest available segment of 7"-9" Hart & Cooley
chimney pipe, take it to a sheet metal shop, and have a sheet metal
cap custom made, which could then be attached the the installed pipe.

b) much simpler, less expensive and more practical would be to cover
the protruding rain cap FROM ABOVE with some flexible waterproof material.
Initially I thought of a cap of rubber, then of tarpaulin, then of 
plasticised fabric as is used for sleeping bags or tents fitted over
the rain cap from above as snuggly as possible, and secured at the
roof shingle plane with 1) nylon rope, 2) #14 insulated wire, or
3) a suitable steel clamp. Now it occurs to me that a conventional
one or two gallon plastic food storage bag, especially if doubled
or trippled would serve the same purpose, and being colorless would
not be apparent from the ground. Admittedly there might be maintenance
problems, analogous to the annual cleaning of gutters. The fabric or 
plastic might break down from sun, wind, and rain exposure and require 
replacement after several years. Need for such replacement would become 
apparent by water staining of the ceiling plaster inside the closet, 
no different from the effect of a leaky roof from any other cause.

c) The attempt to arrest drainage through the Rain Cap from below seems
to me impractical, not only because of inaccessibility after plastering,
but also because occluding the 7"-9" chimney pipe from below would 
potentially create a pool of water which would not evaporate and which
could not be drained.

==> If you plan to go to Nantucket BEFORE Nov 11, please advise me of
the date, and whether you plan to take the car or the hand truck, so
that I can assemble the shelving, the PCV cap and clamps, the Rain Cap
cover, and whatever else you wish us to take along.