If I was making one.. I would use a length of 3/16" brass rod and just turn down the end to 1/8" and put a thread on it with a die . It was painted in green OD anyway. No one will notice the difference ..it's just a thin tube after all
BTW the coax lead from the 19 set to the B base is a tuned length ..it's worked out as a percentage of the wavelength . The B set was a useless thing in reality ... the crews hated the thing as it was more of a hinderance than anything.