Another concept you can do to hold it in place (we did it last year and it worked perfectly) - build a little "seat belt" for it. Simply buy a buckle like this:
, work in two short nylon straps, and you're set! We found that a simple pop-rivet was plenty to hold the ends of the belt in place, and with the buckle right on top of the battery it was extremely easy to undo when a battery needed to be replaced.