Balance Board
DIY
Cut out some rectangles out of 3/4 ply, I did 36 inches by 12 inches.
round those rectangles. I used a 5 gallon bucket to draw a half circle.
Cut out a bottom arch 24 inches long and 3 inches tall at its tallest point.
Cut out to skinny arch halves about 3 inches by 3 inches. 1 quarter of a circle.
Router the edges to make smooth edges.
Sand Sand Sand.
place the arches on the bottom side of the board and drill out the holes so that the screws you have go into the top board about 1/2 inch. (use multiple size screws for more support)
Router the edges to make smooth edges again.
Sand Sand Sand.
Paint the top if you like.