It took so long that you might have thought that I was bluffing when I first mentioned it, but I finally got my canvas pencil case made!
It was made possible by a small, local art supply store, Rath Art Supplies. I went in to buy a Col-Erase colored pencil for drawing, and in the "free" box there was some lovely canvas just the right size. It already had a coat of gesso on one side so I was all set.
First, I applied a glazing of acrylic paint:
After it was dry I added the designs:
Some lining and a zipper, and voila!
I got the idea from Alisa Burke, who sells similar pouches in her on-line shop. Unfortunately, she doesn't ship outside of the US, so I made my own.