The S parameter plot for the two loop probes is attached and the match is not as great as it would seem.
I spent most of today pricing out vacuum pumps and looking at vacuum enclosure designs. I have attached some of the designs I have been looking at and the round one is where I am starting. If I can get some free welding help from friends, I might try for the square one. Whatever the design, it is going to have a clear cast acrylic window to see inside.
Do you know of anyone in the university that has a large vacuum chamber? It would have to fit the 15″ (390mm) long by 11″ (280mm) diameter cavity and include a N to N-type connector in wall for the input.