Yeah, macbow is pretty much right.
Just find any handle that works.. then make it your own. I used the Binghams design then radically changed it. But it gave me centers and the pad angles.
The riser can be many different lengths.. 12", 16", 20".. depending on what overall length and the type of pull you want. So your block of wood (woods) will be approx: 2" x 4" x 20" then go from there.
If you do some searching on here, you can probably find someone that has posted a design drawing.