On my 300B amps the cages are designed to be about 1 inch higher than the 300B tubes. There are only two other tubes in the amp...a 5AR4 and 12FQ7. What I'm saying is that it is not exactly an amp that runs hot in the first place. I can comfortably place my hand atop the cage and hold it there after many hours of play, The cages are made of blackened perforated steel all around so have very thorough ventilation. YMMV with the hotter Rocket 88 amp, but I have no doubt whatsoever it will be a bit improvement over having the raw tubes exposed to touch. You could also use aluminum mesh which would likely disapate the heat better. Again, even with steel, I'd bet you would not burn yourself on it as long as it was perforated (or mesh basket weave) all around.
Marco
Marco