Because of all the waste, I find it cheaper and easier to just buy the boxes, unless you have a good source of free or cheap scrap lumber.
A standard deep box is 9 5/8" deep (and 16 1/4" wide) A standard frame is 9 1/4" deep. That leaves 3/8" beespace. If you cheat that down to 9 1/2" and the frame down to 9 1/8" (as many of the manufacturers have done) then you still have 3/8" beespace.
Mine are all 6 5/8". For some reason they never cheat on this. Also, your lumber comes out much better with a good inch less waste.
And the boxes are lighter. :)