In 2009, I won the Instructables Tap'd NY design competition with this project.
-A board of wood or plastic at least 0.5 inch thick (no fiberboard)
-A plastic rope
-16 Pan Head screws
-16 water bottles and their caps
-Paint (the color you want)
-A piece of flexible foam (1/2 inch thick and more), like a square of Play Mat (I took my little sister's)
-Teflon tape or silicone tube (optional)
-A hot glue gun
With a drill or a punch, make a hole (the size of your screw in the middle of the HDPE caps. Do not use a nail, it would stretch the plastic.
2- WOOD CIRCLE
Cut a circle in your board. The diameter should be about the size of your foot. If you don't have feet, (sorry) make that 12 inches. You can sand or cut 16 equal flat surfaces around the circumference. Drill a hole in the middle, the size of your rope.
Drill 16 holes at equal distance. Make sure the drill is in line with the center of the circle.
Screw the caps to the center piece. (Adding a drop of silicone in the middle of the hole will it make is water tight longer)
Use water resistant paint.
Cut an 11 inch circle in the foam board. Make a hole in the middle (two times the size of your rope)
Glue the foam on the board. (Perfectly centered please)
Make a tight knot in the rope after passing it through the hole. Melt the strings together (do NOT breathe the fumes).
Find something heavy like a rock or a brick. Attach the end of the rope to it and trow the weight at the botom of the lake. You might have to adjust the other end of the rope so the buoy isn't too loose.