I am building a 12' X 6'6" X 12" riser. I would like to turn this into a bass trap. In order to turn it into a bass trap, I will be adding vents to the back of the riser. I am also building a 4" riser on top of this one just for the seats. This second riser is where the vents should go. Should I leave the back of the 4" riser open and have the vents there? Is there another way to do this? Thx