Although the finned heat shields are excellent at moving heat away from a
particular tube, they do not remove the heat from the cabinet. Heat still
remains in the cabinet. The best long term solution is to remove the heat
from the cabinet. Often, this involves a fan running at reduced speed (for
the sake of noise). Some of the modern small computer fans would do
wonders.

That may require cutting holes in cabinets. Some people are not willing to
do that. Little things can help. Reducing the line voltage fron 122 volts
to 110 or 115 will reduce heat a bit.. Even a cabinet that encourages
natural convection flow will help.

Regarding the tube shields themselves, shiny material causes the heat to be
reflected back to the tube. So, black inside and outside can help - even if
painted. Having the black finned material between the shield and the tube
can help.

You can purchase an infrared heat sensor and measure the temperature of the
tubes. Also measuring the temperature of the case will tell you if you are
making progress.

I have found that reducing the internal temperature of equipment that tends
to run warm has greatly increased the reliability and reduced tube
replacement to a minimum.

73, Colin K7FM