I would suggest at the very least doubling up on the 5/8". I only say that because I am QUITE sure that once you build it, and you run that "PYLE" subwoofer for a while, you will be itching for an upgrade. If you just double up on it or just make it 3/4" thick now, you will just be able to swap out for your new sub and return the PYLE to the trash where it was properly placed the first time. And not to sound sarcastic, but honestly, you will have as much in new MDF and other supplies in building a box as that PYLE sub is honestly worth. Honestly...