I have a small trunk and wanting to have my two 15' kicker vx in a vented enclosure. all the vented enclosures i have seen have been in a box shape. I am wanting to use all possible airspace in the trunk, so i would like to fibeglass the box on top of the sparetire area and utelize that space. can i do this or do i need to just make boxes? also to find the vehicles reson freq i should use my subs while there in the trunk with a sealed box to determine it?