Thanks for the replies! The city trailer seems like it may work, but I'd have to rig it a bit to handle a 4.5 ft long load.

My fiberglass cello case has backpack straps.. but my center of gravity raises considerably when I try it on a bike, and it doesn't really leave room for my helmet, on top of limiting my ability to look.

Foldable two, I have considered leaving my current bike and single wheeled trailer locked to the bus stop, but some days I have to travel 4-5 mi in town once I get off the bus, so I'd have to look at the bus schedule or just walk it, which may not afford me time to get those things done. I do take my "performance" cello everywhere.. I don't have the luxury to have a practice cello right now. In fact if I did have a practice one, they have a collapsible design that compacts to the size of a violin case! Or I'd use an electric cello. Either would be considerably less bulky.

I actually forgot about the Burly Travoy, which also folds down
http://www.burley.com/home/bur/page_416/travoy.html

This guy can attach to a rear rack, which reduces its angle from the ground, allowing a lower profile for the cello. Problem is I can't find any place that carries it in my area, and I don't know if the bout of the cello case would fit between the wheels and the bottom lip of the trailer.