From the Kirby Museum blog:
In 1969, Sheldon Feldner contacted Marvel Comics, asking if one of Marvel’s artists would be interested in designing costumes for a production of William Shakespeare’s play Julius Caesar by the University Theatre Company at Santa Cruz at the newly-built Cowell College of the University of California at Santa Cruz.
Enter Jack Kirby, at the height of his powers. See his sketches, including photos of two of the completed designs here.