So I am confused... did you use the stock "box" in the fender and then build it out to fit a 13.5" sub? Seems like the volume for the box would then be SUPER small for the size sub. But definitely like the way that you handled things.
Someone else posted about 13.5 sub. This sub is 8" and I removed the stock box and insulated the whole interior so the effective box is now at least 2 times the size of the stock one. Also all the interior panels have a couple layers of sound deadening on back side.