I am also a fan of Herbiesaudiolab.com.
However, if Herbie's will occupy too much space around your "caged" tubes, then check out
tweekgeek.com
for
Marigo Audio Dots
which are placed either on the tube base, or tip of key pin, for resonance control. These add virtually nothing to the dimension of each tube, and will allow for tight clearance locations.
Also, Herbie's proprietor, Steve Herbelin, may be able to suggest identical placement of his own tiny dimension Grungebuster Dots (a different part of his product line) in a way to emulate Marigo Audio Dots.
May the better dot win!