Thank you, Colin: now I can prove that at least _one_ person has read the article ...
... you are 100% correct: I tried the 12x16 holders (that I use for the 12x16 Ilfochrome paper, they are the 14x17 holders internally reduced to 12x16 in the way you so clearly describe) and I can fit a 12x17 film in them.
Nick, I can get some X-ray film from a doctor friend of mine, but I was unable to find an on-line supplier in Europe. I am aware of an on-line supplier in the US that sells 14x17 X-ray Fuji Film but I never tried it.
Has anybody ever ordered through Quad City?