[TerryMason]

Using a pair of vice grips, I pressed the Tee-nuts into the speaker mounts. It's a good idea to put some glue on the Tee-nut backs before pressing them in. It can be a real bear to get them into the MDF. I was then able to test fit the subwoofer into the cutout.

I would suggest that you use these tee nuts instead of screws, mainly because they are removeable (you can take your sub out if you need to). Remember that your box will experiance some significant pressure, and a screw may not hold.