I've used a Mortite/Dennis rope caulk top hat (a little cap, with the tip protruding thru...about dime sized) on all my l'il preamp tubes for years....works great. NO Plast-i-Clay as it will melt....only rope caulk found at Hardware Stores. A much tighter lower end (primarily) is the result...a point of inferiority in tube gear vs. SS that is made much smaller employing them.
I've also used Herbie's tube rings......no meltdown after 3 years.
I've only made the hats and used Herbie's on small preamp tubes(12ax7, 6922, 6n1p - which runs hotter than the others) ...having never owned a tube power amp.