Mark, Jason is right on with his discreption....I had the same questions when I made mine last fall. Be advised that the back all seem to be slightly different size. The first one I made missed the depth of the groove, and it would not hold. Took a small 6 in ruler to seat in the groove and took measurements. If you can find the link here, you will see I use the Homer method of cut twice measure once - Doh!! BTW, I used the hobbie board from a local craft store to make mine with, worked perfect after a coat of flat black paint. Good luck!!