I would Glue between the boards and then screw them together all at the same time, you will get a much better seal. After you screw the boards together than start your silicone around the seams. If you do it all at once the seal will be very good, the glues will have a tighter bond and seal because they are some what mixed.
aight thanks....and could you tell me if it is bad to have your subs facing the roof of your suv???
They would sound great. If you have just 1 or 2 then they would sound better facing the rear of the suv.