I don't disagree with building on a metal chassis as original per se and its benefits for hum levels and stability, just with the idea that it would be stray RF proof. Making the amplifier literally RF immune would be a whole different game.
Looking at the original 5-20, there's a skirted holder on the EF86 but no can and not even a skirt on the ECC83.
http://www.r-type.org/articles/art-003d.htm