Balsa block spreading board--This is merely a block of balsa wood 6 x 6 x 2 inches with a slot cut in one face of the block. The slot should be 1/2 to 1 inch deep and just a little wider than the body of the insect whose wings you want to spread.
Platform-type spreading board--This board is a little harder to make, but it does not require so much balsa wood. Also you can taper the width of the slot to do the job of an adjustable purchased board. When spreading an insect, you just pick the proper place along the slot that best fits your insect.
The exact length and width of the board is a matter of choice, but 12 to 15 inches long by about 4 inches wide is a convenient size. However, the distance from the top of the board to the top of the bottom piece should be 1 1/4 inch or more. If you make this dimension too short, the bottom piece will stop the insect pin and hold the insect above the spreading surface of the board.
To assemble the board, place the top pieces on the legs so the width of the slot between the top pieces is 1/8 inch at one end and 1/2 inch at the other end. Nail the top pieces in place, and then turn the board over so the legs are up. Glue a length of balsa wood strip about 2 inches wide and 1/4-inch thick over the slot. The balsa strip should be long enough to cover the entire length of the slot. Then nail on the bottom piece. The bottom piece makes the board sturdier and protects the pins that poke through the balsa strip when there are insects on the board.