There are several, it's been asked several times. Check out the CAD threads on the main forum and software forum. Also people have used programs like viseo.
I just used a pencil, ruler, large piece of cardboard, and paper cutouts to scale of my parts. Yeah, it's the arts and crafts method of figuring it out, but then you also can sorta tell if it is going to be comfortable. What looks good on the screen may not feel good once implemented.