[Arran]

The thickness of the inner wall of the copper pipe is negligible and won't affect whatever you put in there in terms of electrolytic capacitors. I was just pointing out that copper pipe is sized by standard inner diameter and the parts, like caps and nipples, that fit it are made to fit the exact outer diameter. The Mershon condensers were likely drawn out using custom made dies with a press so they did not use off the shelf pipe dimensions, inner or outer diameter, or wall thickness, the same is true with any aluminum can condensers.
I had to join a pair of aluminum condenser cans back together that someone had crudely cut in half, they were originally made by Solar I believe. With the first one I ended up using a PVC plastic coupling for 3/4 in inner diameter plastic pipe, it fit the two pieces together on the inside, it wasn't an exact fit, but some hot glue made up the small difference. For the second I will also use a fitting, a larger barbed plastic fitting used for sprinkler pipe, but in this case I will have to get it turned down to fit inside the second can.
There does not appear to have been any rhyme or reason for the dimensions used on electrolytic cans, the first can originally had a single cap inside, the second had two caps inside so it was larger. So it was just luck that I could use a PVC coupling meant for the outside of a pipe
to couple a 10 MF can on the inside without much work. So as far as I'm concerned you may as well make a fake Mershon and stick it in your set, while the dimensions may be different I'm sure that the Sprague, Tobe, Aerovox, Mallory, Solar, or Cornell Dubilier all differed from one another as well.
Regards
Arran