I used map sheets from the Forest Service - expired maps of course for my shotshell patterning & developing loads. They were 4' square, thus fairly easy to place the lexan patterning circle over them. I had the 20" centre cut out and with the clear lexan, was quite easy to place it perfectly centered on the pellet pattern, as well as to see if the gun was shooting true with both barrels in doubles - many don't put both barrels on the bead.
About the time I was finished with my SXS 10 bore AYA and 12 bore Browning O/V, the trap section put up a patterning sheet of metal, however it was only feet square and undersized. Either painted, or greased (axle grease) works for taking pictures.

Using the patterning process, I was able to get my AYA 10 bore shooting 94% right barrel, 96% left barrel at 40 yards. The right barrel was quite even, but gave a good good pattern with #2 lead to 60-65yards. The centre concentration of the left barrel was w/heavy and killed greater Canada's to a good 80yards. Then, lead shot bans and I sold both of my waterfowl guns.

To get the gun to shoot that well, I had to re-work the end-chokes as it was too tightly choked.