Shotgun sighting is usually taken with the face over the breech, seeing the rib full length. Much will depend on your fornt sight height, loads and where the gun actually prints.
A 16 bore ball - or rather a .648" ball or close to what is normally used in one, is just over 400gr. weight. A 16 gauge ball is, well, 16 to the pound - 7,000 grains divided by 16 = 437.5gr. to be exact. I've always thought of 16 to the poiunt being .662", like the Lyman mould, however my chart says 16:1 are .663" in diameter - close enough 1/2 thou per side.
You make the paper cups. Plastic merely adheres to the barrel walls after the black powder flame melts it on the way out and sprays it on the inside of the tube - you don't want that. Post-it notes work well and are easy since one side has a glue line. Formed arund a dowel, you can snip off the correct length after folding a base over the end and tapping that to seal it. Anyway, that's how I'd make them. Glue sticks also are handy.