I have been using something as simple as a 6" paper plate. I get 100 of them for a $1.00. I can tape that white paper plate to a brown cardboard box and they show up real nice. I then can put a sticky dot on the center and the very middle of that plate. Also there is an inside ridge and thatis 4 inches.

Another simple target is go to Wal Mart and gets some multi colored 3"x5" index cards. Then draw a X from corner to corner and tape that to a box. Again, it shows up real good you have the X to shoot for the dead center, and if you can keep them on that card, there is no animal that could stand up to you.

And I figure for extreme ranges I can put a fill sized paper plate to a cardboard box and that should be a good target as well.