Here are some pics.
You can get away with a smaller collar if you just use pin locks for the hump. Chicompany sells short pin lock connectors as well to save even more space, or you could just convert the pin locks to ball lock.
You can also purchase "shorty" 5 gallon ball locks, 22" tall and 9.5 diameter That's what I did so that I could fit one on the hump without changing my collar. http://www.homebrewing.org/5-Gallon-Low-Profile-Keg-Used-Short-22_p_3545.html
Primary - Citra IPA, Boston Red Ale
Kegged & Drinking - White House Honey Porter, Fat Tire Clone, Deadman's Curve Black IPA, Chocolate Peanut Butter Stout
A "shorty" five gallon "ball lock" keg is undoubtedly a converted pin lock keg...