[asifnyc]

I assume you mean bolt/screw through the bottom. If I use bolts the battery would wind up sitting on the bolt heads... I could countersink some sheet metal screws but would that be strong enough? also not excited about a bunch of pointy screw tips on the underside of my trunk (mostly because I assume I'm going to cut myself on them at some point in the future...).

I could weld some threaded rod to the bottom of the battery box and bolt it in that way...