Once you have a nice strong shell get out a dremel or grinding wheel and cut off the feathered glass edges and any runs that occured on the outside of the shell. An angle grinder will eat fiberglass MUCH easier than a dremel but use finesse. Now cut the woofer ring from MDF, you can get the dimensions from the speaker manfacturer. I stacked 3 rings to get the strentgh and projection I wanted. Using dowels and hot glue(and an extra set of hands) measure the dowels to support the ring at the desired depth and height. These dowels are not REALLY structural but will need to adhere strong enough to support the tension of the felt that will be stretched over it. Test fitment in the vehicle again.

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