fg sub enclosure in a boat

I recently decided to add a 12" sub and some 6 1/2 full range speakers to my 1995 18' SeaRay bowrunner boat.  I decided to put the sub in the fiberglass floor under the dash as the floor angles upward under the dash at about a 45 degree angle approximately 16 inches.   So I cut the mounting hole for my sub and removed as much of the foam under the fiberglass as was practical, leaving me with about 1 cubic foot enclosure.  Here's where the problem is.  In this area is a fiberglass brace that sticks up into the enclosure leaving me about 2-3 inches shallow on mounting depth.  I am going to build the enclosure outward  with fiberglass to give me more volume and mounting depth.  So, I was wondering if I would need to enlarge the original hole cut out of the fiberglass to make one large enclosure, or would I be alright just building up from there and leaving the 11" hole as is and kind of have two smaller enclosures joined by the original mounting hole, if that makes sense. 

Why don't you just make the enclosure smaller and add some Dacrom filler ( this fools the sub into thinking the enclosure is actually larger ).Jeff

Velocity Custom Home Theater

Mobile Audio/Video Specialist

Morden, Manitoba CANADA

I would do that, but unfortunately, there is a fiberglass brace running down the middle of the hollowed out enclosure that is putting the mounting depth for my sub about 2-3 inches too shallow.