These guides are extremely easy to make. I'm really posting this instructable to share the concept, since I have no doubt you are all capable of drilling some holes in a piece of plywood;). Three common spacings in an SFG are 4, 9, and 16 plants per square foot, so we will build our guides to those dimensions. To make each one, I first divided the blank into a grid of equally sized squares. The dimensions of said squares are as follows;
-4psf guide; Grid of 4 6"x6" squares
-9psf guide; Grid of 9 4"x4" squares
-16psf guide; Grid of 16 3"x3" squares
Last but not least, I drilled a 1 3/4" hole in the center of each square on the grid, and the guides were finished.