Is there any kind of a venting system? All batteries out gas and therefore if the battery is not vented properly, hydrogen gas could build up in the trunk and explode. It could have been a defective battery if the battery exploded from the inside. How big of an explosion, whole trunk or just the battery? If the battery exploded internally, I would demand that the manufacturer pay for any damage and of course a new battery.
Make sure any acid in the trunk is neutralized or later you will find damage that isn't visable now. The acid will slowly eat thru the trunk upholestry.
There are disadvantages in placing the battery in the trunk. One is the problem you had and another is the battery gasses geting into the trunk if not vented properly. Another problem is the long cables necessary to connect the battery to the starter and the resultant voltage drop.
The advantage is the battery does not get as much heat from the engine which should increase the life of the battery.
P E H