Do the math. Measure your bore accurately using calipers. A nominal 20 ga. is .615" in diameter. Multiply that diameter times PI( 3.1416) to get your circumference. ( In a nominally sized bore of .615", the circmference is 1.93") Now divide that number in two, as one strip is going to form both "sides" of your cup, as well as the bottom. ( The nominal bore will use a strip that is .966" wide, or 1" for proactical purposes.
Since all fabric comes with "sizing" in it to make it iron flat for compact storage and transportation, you must wash the fabric twice and then dry it, and iron it, to prepare it to be split into strips.
For that reason I do NOT recommend asking fabric stores to cut the strips for you. Too much fraying of the edges will occur during the washing and drying.
Start your cut with a scissors, and then use a knife stuck into a wood surface( with a sharpened blade) to cut the strip s as you pull the fabric towards you passed the edge of the knife. Once you get the hang of it, you will mark the cutting edge of the fabric with a pencil using a ruler to measure off the intervals, and then efficiently cut the strips one after another.
A little overlap of the fabric in the barrel is not going to adversely impact your patterns. It may cause a larger "kink", or fold, at the bottom of the cup, but if you use an OS card under( behind) the cup when forming it, and then seat an undersized OS card inside the cup to add strength and provide a slick surface to push the shot out of the cup when it leaves the muzzle, the shape of those kinks or folds will mean nothing to the patters you will get.
Paul