you could use a crapload of super sticky double-sided tape, and i mean a lot. I cant take responsibility for it if something goes wrong but you could at least consider taping it down to a freshly washed surface with some sord of permanent adhesion. I still wouldnt reccomend doing so without some sort of emergency-strap though because something as heavy as a battery can be VERY dangerous in a vehicle if said object decides to break free and move about the interior. depending on how fast you are going the battery's weight can multiply several times over and then if it hits you...you're toast.
better idea of mine....
take out your spare tire and strap it down in there... there is already a threaded hole for a bolt and it's recessed so you will have a lower CG than if you mounted it above. you wont have a way of getting home on a flat but if you want your weight distributed better that much, it might be worth it to you. you could then maybe rig some sort of permanent tie-down system or even replace the stock plywood trunk liner and it would serve some protection for keeping the battery down if the bolt were to break. that is...if the battery will fit. if it does you wont even see it or loose luggage space
oops...i just re-read and you might have mentioned you dont want it there? oh well...i tried.