I take a piece of flat cardboard about 3' X 3' draw lines that cross, and draw a circle in the middle. Go back 27 yards and aim at the circle and shoot 3 shells. Done. You can also find old blue print papers at construction sites and use the white side. All free.
At my club here in CT, they have a steel plate set up on a target stand and a paint roller coated with heavy grease. Grease the plate well, step back to the line, take your shot, go count the marks... Works amazingly well with a minimum of maintenance.