The SmartBox Fan Box is a good solution when you're up against the side of a Joist, and it can be installed without going in the attic:

[Linked Image from licensedelectrician.com]

It's a plastic box, - it doesn't show in the picture above, but it has a hefty metal bracket up the joist side and across the top:

http://www.licensedelectrician.com/Store/SB/Images/SBFAN_Release.jpg

more info:
http://www.licensedelectrician.com/Store/SmartBox-3.htm

Bill