Ive been using the small ones, 30 cal?, as speaker boxes for about 10 years. They work great. The ones I'm using are about 4 - 5 inches wide. They sit on the rear fenderwells just about where an arm rest would be, right beside the roll cage. I left the handle on them and it gives the passanger a place to grab. They are water tight, and with a hasp on the flip up handle, they are lockable.
They do vibrate some if you crank it way up. but at 60 you can just barely hear them anyway. they also produce a tinny sound unless you stuff them with something.
To fix part of the vibration i took a piece of 1/4 in plywood and cut it to fit behind the slim line speakers inside the box. I usually carry towels, tp or a parka on the speaker side of the plywood. this seems to dampen it quit a bit. I had thought about porting them to relieve the pressure but then they wouldn't be waterproof.... resistant.
I use the other side of the plywood for storage. To give you an idea on how much stuff you can cram in them, heres the list of what is in mine all the time.
One has pliers, cresent wrench, screwdrivers, tire patch stuff, a dist cap, points, rotor, a spark plug, bailing wire, elec tape, about 10 ft of 10 ga wire, and fuses.
The other one has registration, insurance, Fire permit, forest service map, Gloves,Parka, 2 Trash bags, Flash Light, P38 can opener ,Matches, sun tan lotion