Quote from: "enjenjo"6" by 9". The speakers are mounting in the rear corners of an AD truck cab. There is limited room and I did not make the first try deep enough to clear the magnets on the speakers. I was able to recycle the first try for the front speakers that were smaller. I used 1/8" ABS plastic lined with acoustic foam.
Here's a trick that you may have seen, to have the speakers "see" a larger volume box to improve the base response; insert a fair amount of poly-fil into the boxes. Pack it loosely, leaving enough room to easily insert your speakers into the boxes.
Available here, sewing stores, or your Wife may have some:
https://www.amazon.com/Fairfield-Poly-Premium-Fiber-32-Ounce/dp/B08Y2HRQ5S?th=1