With 2 kegs, i can fit my 5lb and reg on the hump no problem. A 5.5 or 6 gallon bucket from my LHBS with an airlock will fit on the hump with less than an inch of room on top, but needs to be supported so it doesn't tilt forward. A 7.9 gallon bucket from Austin Homebrew fits well, and is actually shorter than the 5.5. The whiteboard I used inside the door is a little thicker than the original shelving panel, but the screws have enough thread to grip tight. I built a mini paint can heater out of 8oz paint can, a candelabra size socket (old christmas light size), and a 7watt bulb. That is enough to keep the fridge between 19.0C and 19.5C with 16C ambient. I started with a 15watt bulb, but the can gets too hot to hold and I was afraid it could melt the plastic it sits on. I'll be adding a fan to the base of the heater to dissipate heat quicker.
I'll send you pics later of different configurations.