Joe Harmon, 35, an industrial designer from Mooresville, N.C., on the Harmon Splinter, as told to A.J. Baime.

I’ve wanted to design and build my own car since I was a kid. I was studying industrial design in graduate school, and I realized this was the time to do it. I wanted to take building materials—things like wood and glue—and do something with it no one else has.

I...