Hi Vince:
The 1/4" board was most definitely for the back of the box hopefully preventing more signal escaping out the back sheetrock. It is different when doing retrofit versus new construction. Fiber fill can be more important for retrofit allowing you to tune the enclosure. The more fiber fill you add the more the speaker thinks it is working in a larger enclosure. The fiber fill really absorbs the low frequencies. I wouldn't worry about reinforcing the front. The eight dog ears will provide a tight seal. Don't forget adding a bead of Silicone on the back of the front bezel/frame. You could apply the Silicone around the cutout hole instead. Thank you for the kind words. TL/12/3/20