I have previously discussed making shotcups using 3M Post-It Notes.
First get a mandrill- metal, or wood, it doesn't matter- that is several thousandths of an inch smaller than the bore of your barrel.
Second. I found that two(2) complete wraps around a mandril made a strong enough cup that it would not tear inside the barrel. This is, of course, partially dependent on the relative size of the finished cup compared to your bore diameter, ie., the larger the difference between cup diameter and bore diameter, the more likely the cup will tear inside the barrel.
Third: I use a next small gauge card or OP wad inside the bottom of the tube to help form the cup and keep its shape. I put the wad inside the tube, and then fold over the end of the tube behind the wad. I then dip the closed end of the tube in liquid parafin wax, and then push the closed end down on wax paper I put on the counter next to my stove.( Heat the wax in a double boiler arrangement, using a small tin to hold the wax, sitting in a small frying pan with an inch of water in it. The water keeps the temperature at or near the boiling point of water, and well below the flashpoint of the wax.)
Fourth. It only takes a few seconds for the wax to harden-you can both see it, and feel it through the mandrill. I remove the cup from the mandrill and leave it on the wax paper to cool while I make another cup.
The adhesive strip on the Post-it Note paper will seal the tube together if you orient and trim your paper to the correct length needed to make those two complete wraps around the mandril. That, frankly is why I chose to try using the notes for this purpose. It saved me one PITA thing I would need to do to make a paper shot cup!
I don't like using celophane tape to seal the cups. Its rubs on the barrel, can melt and leave a sticky carbon residue in the barrel, that only a good lead solvent, or acetone will clean out easily. The wax I use to seal the bottom of the cups holds well enough, and the wax is deep enough to reach up about 1 inch above the bottom, providing some wax " lube " to the sides of the cup to allow the cup to slide out the barrel. If you then lube the barrel with a greased cleaning patch, after loading the OS cards and shotcup down the barrel, the lube in the barrel protects the bore from rusting during a long day's hunt, provides grease to allow the cup to slide down the barrel, rather than rubbing lead pellets against the bore and leaving lead deposits, while leaving enough lube in the barrel to soften powder residue to make it easier to clean the barrel before reloading it.
Fifth: You will want to trim the top of the cup away at the muzzle to match the length of the cup needed for your load of shot. I Leave the tubes long enough to allow me to vary the shot some, if I wish to increase or decrease loads in the field.
Sixth: To open the the cups and release the shot, you will want to cut slits in the sides of the cup, How many and how long they need to be will be up for you to learn depending on how tight a pattern you want. All I can tell you is that if the cup does Not open up, you are likely to get either a " slug " -- ie, one hole in the pattern board--- or a cluster of shot smaller than your fist at 25 yards.
That may be okay for self-defense purposes, but it will ruin any bird hit. If you don't want the cup and the wad inside it to penetrate patterning target at 25 yards, you want to use the OS cards, rather than the heavier OP wads in making the cup. AND, you definitely want to cut slits in the side walls of the cup to create an " air brake " to stop the cup's forward motion to help release the shot as soon as possible after it leaves the muzzle of your gun.
