I currently use a Polaris ATV battery in my M-IV. It fits in their nicely, but it too tall for the factory bracket. I use two of those black rubber bungy cords, and a tie down rope. Holds the battery down very well.
Due to the positive being on the opposite side of the previous battery I had to extend one positive cord to reach over to the other side. I also had to move the flat, braided ground wire to the opposite side of the frame as well.