Michael Mutmansky wrote the article as he tested 7x17 holders. He was using a 7x17 RH Phillips camera which he has since sold. He was the coach or reason I bought one. As did John Bowen, after reading the article, I got to work with the sharpies, even took the wooden piece off where the film slides in and painted inside there on either side where the film passes.
Most of the time in real life the holders are out of the camera. Most of my 7x17 holders are in individual fabric cases sold by Quality Camera on eBay for $29.95, plus postage each. Quality Camera is in Atlanta, GA USA, tel 404-881-8700. The cases have the name MC Photo on them. As with everything Large Format these add up in cost. It might be much cheaper to buy one and have some seamstress locally make the others you need. A cheaper alternative is the black plastic bag 16x20 print paper comes in within the box. This is a commonly used size for 7x17 because you cut it in half with a couple of test strips left over for contact prints. I am assuming 20x24 print paper also comes in a larger plastic bag that could be used for this. Besides keeping light out any bag or cover also helps keep out your other enemies; dirt, dust, grit. Most of us carry multiple holders in some sort of bag for easier transport and additional protection from the enemies.