I first explored the opportunity of using a toilet paper shell for this but the problem is that a shell is too large to stay on the terminals and to reduce it inner diameter, I would have to put lots of some soft material like foam inside it, and I didn't have any foam handy.
Then I made the decision to make the caps entirely out of duct tape and paper. The duct tape must be 50mm wide so that its width is significantly larger than the length of the protruding terminal part.
Firstly, cut one long length of duct tape. Align it inverted on the duct tape roll and attach it to the end of the roll in as good alignment as you can. Then cut the duct tape roll again. You will have two opposing duct tape pieces attached to each other:
Then wrap the thing around itself around the protruding terminal part, in as good alignment as you can. You will then have a battery with its protruding terminal part covered by a duct tape shell that's exactly the correct diameter to cover it. Cut any exposed adhesive surfaces (due to misalignment) from the top. Cut a 50mm x 50mm duct tape square and cover the open end above the protruding terminal part with duct tape.
Now you have a shell with one end open and one end closed. Remove the semi-open shell and finish it by cutting any small exposed adhesive surfaces (due to misalignment) from the bottom.
The problem is that the closed end has an exposed adhesive surface inside it. It shouldn't matter much but if you consider it a problem, you can eliminate it in this manner: cut a square of paper slightly larger than the protruding end of the battery terminal part. Fold its sides so that it forms an inverted cup on top of the protruding end of the battery terminal part. It's easiest to do this on top of the battery, using the battery as the model. Then place the inverted cup on top of the protruding end, and slowly slide the duct tape shell around the battery terminal part. IF you do this carefully enough, then you can remove the shell upside down and see that the paper piece is well-aligned but doesn't yet touch the adhesive surface. You can use a small inverted screwdriver so that the handle of the screwdriver is used to gently push the paper to touch the adhesive surface.
(About why you have to fold the sides of the paper: if you don't my experience is that the paper never stays straight when sliding the shell, so that you have a shell with a paper on one of its sides, not where you want the paper.)
Here's the finished product:
Note that there are no large exposed adhesive surfaces. The only small exposed adhesive surface is two to three millimeters wide and could easily be cut away with scissors should it be seen as a problem.
I tested that holding the battery upside down, the cap is lightweight enough and friction is large enough to hold the cap on the battery terminals. It's easy to remove in about two to three seconds and reattaching takes about 5-10 seconds.
Four such caps took less than an hour to make even with lots of trial and error. I'm sure that without the trial and error, if I had these instructions already, it would have taken about half an hour for four caps.