If this is just a one time deal then the poster board or a big cardboard box will work. I test my POI vs. POA at 16yrds with a full choke. I shoot while "aiming" as best you can with a shotgun at a spot on the board about 3-4 times. I just use birdshot for this. This will show you the average impact and help eliminate errors with just a single shot. I like a field gun to have the pattern average centered left to right and what is called a 50/50 pattern. Half my pattern hits where I'm aiming and half hits above the point of aim. If your going to testing differnt load and choke combo's I went to lowes and bought a roll of brown paper from the painting section for $10 and just staple it to a wooden pallet.