The Monoprice speaker thread, at AVS, has lots of good info on retrofitting speakers, in general.
Flexible back boxes, made of a vinyl-like material, (
Dynabox and
Russound Flexbox) are available for retrofit, but I doubt they do much for bass. They're designed to improve performance, and diminish sound transfer, but the specs I've seen show they're only good for reducing transmitted high frequencies and not bass. The Russound product is fire rated (not sure of the time), and is designed to be used in a garage - but check the rating, again not sure about it. Brandon at AutomatedOutlet gave me the tip on the Russound - I think they were half the price as the Dynabox, very affordable through AO, if you CALL by PHONE.
Stuffing fiberglass batts above the speaker doesn't do much, as the sound travels around the fiberglass, through the joists, right up through the floor above. I tried it in my wiring closet ceiling - no sound difference in office above.
A CI at remotecentral.com suggested using a plastic grocery bag around speakers, in an attic, with blown-in insulation. It was quick and easy to build a back box, so I did that instead, for my MBa speakers.
Most speaker manufacturers will tell you the ideal back box volume for each speaker model. Contact them.
For my garage speakers, I'll prob just buy fire-rated speakers, with an enclosed back. Easier than DIY.
Edit - Russound Flexbox is 1 hour fire rated. I don't know if that's kosher for a garage, you'll have to figure it out. If someone does, please post back. B)