Chris- There is another option- I've used a Costco retailed Bell trailer ($139 a couple years ago, like this- http://www.epinions.com/bike-Compone..._Bike_Trailer#) to pull my #40 son (short commute on pavement to school) as well as some fire road mtn. biking- Worked fine for both, and would surely fit a hard shell case.

I also use the Bell trailer on a fishing site commute via gravel road - 6.2 miles uphill with 25 # of fishing gear. piece of cake, and I'm not that fit. Seems that your guitar commute wouldn't be too bad with a cheaper trailer than a Bob, if you're so inclined