Lay your plywood flat and position you bucket upside down on top of the plywood. Measure the distance from the edges of the plywood to the bucket and center the bucket by making sure all these measurements are equal. Trace the circumference of the bucket using your pencil.
Turn the bucket over and center it on the plywood using the same process as you did in the previous step and trace its base.
Draw two lines from corner to corner across the plywood. Where they intersect is the center of both of the circles. Measure the distance between the two circles and draw a circle that is spaced equally between the two. Do this by tying the string around the pencil and holding the other end and the intersection of the lines.
Drill a hole between the smallest and middle circle so the tangent of the hole is against the inside of the middle circle. Cut along the line of the middle circle using your jigsaw. Your bucket will rest in this hole.
Set two 18-inch boards on edge, parallel and 15 inches apart. Set two 15-inch boards perpendicularly between them, on edge and 15 inches apart. Set the plywood on top of the boards so the edges are flush and screw it to the boards using three screws for each board.