I wanted my instrument to feel similar to a violin. I found dimensions for a violin online, then made a silly looking sketch of a fiddle with some useful numbers on it.
all members of the viol family (violin, viola, cello, contrabass) are built similarly and function similarly
I started the project planning to use an olive oil tin as a soundbox. I didn't like how the tin sounded when I tapped on it, so I made a wooden box instead.
I didn't want my instrument to have as many curves as a traditional violin. I had seen pictures of teardrop shaped folk fiddles in the Foxfire books, and I read about a famous trapezoid fiddle made by Felix Savart. I decided to make my fiddle's box trapezoidal also.