Materials to reduce sidewall flex & vibration ?

Thanks rotarius. I very much respect the benefits of measurement with equipment; but it's not available to me.. You mention " you could use materials that dissipate vibrations in the form of heat on the inside walls without much thought ". If so, why could the same materials not be used on the outside? I'm simply looking for the best material for this. So far it seems like an epoxy with fillers of fiberglass,carbon fiber and lead slivers to both provide rigidity and dissipation of vibrations by turning them into heat. Your thoughts?

Well, for one thing, stuff like Dynamat, norez, etc. would look ugly on the outside! Your aim should be to minimize the excitation of vibration of the speaker walls so the material you use would ideally be placed between the speaker wall and the source of the excitation. I suppose there may be something to gain by sticking them on the outside.  I am just not sure if the effort is worth it all but hey, if you like tinkering like me, that's half the fun.

Thanks rotarius. As a fellow tinkerer I'll let you know that I do consider the final
look to be important and have decided to use good quality leather,in black (speaker color),as necessary to refine the look. I've already got the leather. I've found 3M Super 77 spray glue to be ideal for leather;just the right tack time then a nice permanent bond.