I have a good number of Leica brass reloadable cassettes, either work in the Barnack and, if I recall correctly, the 'M' as well.
They are a bit longer than the modern cassettes, but built like tanks, with an opening system I believe is still accommodated on 135 bulk loaders.
I wonder if they would work in the Leica CL?
Looking at the new types, actually taking one apart, the body looks to have a dead simple metal form, something that a loader could make for themselves, out of thin yellow brass or phospho bronze from K&S Precision with simple hand tools and an cheap arbor press.
The spool and caps could simply caps from regular film cassettes, though it may be that two cassettes bottom caps are needed for each brass body, due to mangled top caps from bottle openers.
Earlier tonight I watched a YouTube video on making a rig for manufacturing 16 gage steel into boxes and a tiny such platform, mostly wood, might prove ideal for making cassette bodies, perhaps.
At the least, take an old cassette and carefully open and stretch it out, and spend five minutes wondering, "Why couldn't I make this for myself.".