Anyway, I wanted to share the end result:
I'm in the book publishing business by trade, so I needed to create a back cover that reflected the contents as well as tying in visually with the front cover. I'm kinda compulsive that way.
I used a yellow cover so it'd complement the red and blue covers of the main rulebook and the companion. (The whole CMYK color scheme so important to printing.) I think they'll look nice on my bookshelf. If I do a Volume 2 down the road, it'll have a black cover ("K").