The prints have finally come in and they look as expected (because I had stared at the CAD model for hours making sure everything was right) and the dimensions are good (1.75mm input holes, even less warping than the r0 print, cartridges slide in nicely), but there's a problem:

As you can see, both prints seem to be like completely blocked. As you might be able to see in the second image, I don't think they're "welded shut" or anything, because poking with a toothpick has put some indentation in the lower right diagonal one. One of the prints is 53.2g and the other 52.7g, so it seems that the quantity of powder in them are different. At least I've got 2 so I know it's not a one-off.

Not sure how best to proceed. The channels have been modelled larger and shorter than Revision0 and yet powder has been left in?

In related news, the box these double c8ors came in was half the dimensions in XY and ever so slightly shorter in Z, suggesting that perhaps the Coaxial8or r0 was just a tad too thick for the smaller box and that's why its shipping was double.

I've also remembered that I should be looking for a gasket material, and was considering brass when I was searching for a copper sheet, but copper gaskets are a real thing (unlike brass it seems):

According to this article, copper was actually "one of the very first materials that was used in the gasket industry".