Looks like a pretty creative idea. I use good ol' wood. But maybe if I had a 3-D printer. . .
One thing that I do is I make the blocks thicker-than-caliber. Using a thicker block allows me to push the ball flush into the block, then run a patch knife across to cut the patch material. It's similar to cutting the patch at the muzzle only I do it on the block.
I don't so much have in mind "fast follow ups" as much as the ability to load without removing my gloves/mittens. Big blocks, cappers, lanyards, they all help. If you can load and shoot without ever removing your gloves it is incredible how much more pleasant winter shooting becomes. I actually like my muzzleloaders in winter better than suppository guns.