Don’t forget that valve amps and radios had been around for several decades before Leo started building his musical instrument amps, so construction techniques were already well established. What wasn’t around were the huge range of plastics we have today. So he’d have used the standard materials available at the time, but better ones now exist.

Fibreboard will absorb liquids and so can become contaminated. Glass fibre won’t absorb liquids to any noticeable degree, so any contamination should stay on the surface and can be cleaned off.