Hi Craig,
No program, but my butts going to be different to yours anyway.
How about sitting in some quick drying cement and using that as a way to mold fiberglass to your requirements.I am a proud Ludite.
I tend to forget the technology because it becomes the center of attention not the real center just a red herring one [your choice though].The actual butt mold will end up being what you're butt actually wants! I am a professional design engineer with many years of bitter experience!
High tech solutions, for one offs,often need reworking,just cut to the chase imho. Unless you are going into production then there's big benefits in what you are chasing.
cheers,