One of the secrets of this subwoofer box design is the woofer bracing,
a famous European loudspeaker builder taught me this important trick.
Put the round braces from sign to sign LEFT to right.
After the glue of the braces will dry,
put a nice amout of silicone between the new braces and the woofer.

see the back of the woofer silicone strip.
Like done at the front side, mold RTV silcoe with of approximately 1 inch width
(or what left between the woofer back and its braces..).
Let it dry and then attach with additional fresh silicone.