The M3 guys have used the Dyna Batt as well as Hawker Genesis. Some can be had for much cheaper.
Some guys made their own brackets out of wood or metal. I think there might also be adapter kits to make it easierl to bolt in the place of a regular battery.
The mounting is the tricky part. The only other thing to keep in mind is your electrical system usage and climate.
I remember one guy got himself two batteries "just in case". He managed to get the Hawker Genesis batteries at such a good price. That way he could keep a spare battery.
Main concern witht these batteries is draining them accidentally.
http://www.stealth316.com/2-dynabatt.htm