I'm about to build a box for two avalanche 18s and i'm trying to decide if i should go with a 3/4" mdf box with a double port and baffle, or should i go with a double layered 1.5" mdf box that also has a double port. The thing holding me back from double layered is that its a 15^3 box and with speakers i'm looking at an easy 500lbs. I drive a 98 tahoe (4x4) and don't know how much 500 lbs in the back will affect it as far as gas mileage and suspension goes. Another thing i'm having trouble deciding on is what i should do about bracing and how much bracing difference would i need with a 3/4 box compared to a 1.5" box. Any help will be appreciated.