I only ever made one "purpose made" patch knife and that was back in the late 70's. I made it from a very thin Indian Ridge Traders' 4" blank that came pre-hardened. Even though I hollow ground that blade on a rough snag grinder, I did it slowly enough that it did not anneal the blade. I only ground it on one side, thinking to leave the flat side that would help it to slide over the muzzle when cutting the patch with a straight bottom blade.
Also made the tip of the blade in what I think they call a "sheep's foot" or rounded down from the back to the edge. With that kind of a tip, it was almost useless as a general purpose knife, though it was scary sharp and cut pillow ticking like butter. A good friend liked it so much that he joked he would not give it back after borrowing it, so I later gave it to him.
After that, I never made another purpose built patch knife. Just didn't see the need when a good belt knife worked.
Gus