This was published in the Comic-Con International San Diego 2014 Souvenir Book this summer.
Ken's art is printed on page 123 in the TMNT section. It is the bottom right corner quadrant.
Disclaimer: The images in this publication are copyrighted by 2014 San Diego Comic Convention and are only being shown here to document Ken's list of published works and not for financial gain. Please see the inside cover of the book for full copyright information. This was not a sponsored post.