[Hengist]

Yes I'm sure metal valves would help, but amazingly it seems that 'transparent aluminium' has already been invented!
"ALON® or Aluminum Oxynitride is an amazing and unique transparent advanced ceramic that is polycrystalline (made from powder) with a cubic spinel crystal structure. In the popular media and in the Star Trek community, it is commonly referred to as Transparent Aluminum. Surmet is the only company globally, that manufactures ALON®."
I can't find any mention of it's screening properties though. My gut feeling is that compounds of aluminium don't behave like the metal when it comes to RF. Does anyone know? (Bound to be too expensive anyway!).