fiberglass enclosure in spare tire

Hello all. I have spent a lot of time readnig this forum in the past but this is my forst post. I have been installing on the side for about 20 years, but have never built a fiberglass enclosure. I had a local shop make the fiberglass corner enclosure in my car and it  is the only thing I have paid someone to do since I was 16. Anyway, I have a friend who just purchased a new Toyota Venza and has asked me to install a system for him. For the subwoofer, we actually want to do it IN the spare tire. (not the well, but the tire itself) The car has the 20" wheel option and the spare is 20" and mounted in a way where the "bowl" of the wheel faces upwards. Nothing goes in this area so it is essentially extra space. Therefore, I have approximately a 18" circle that is 3" - 5" deep. While probably not conventional, seems like a good first project as a; none of the fiberglass will be visible so I won't have to freak out about sanding, bondo, etc... and b; I can do most, if not all, of it out of the car so fumes/small will be easier to control and I won't have to keep his car for very long. He's not a huge basshead so one shallow mount 10" or 12" will be fine for him. Once the enclosure is close to complete and we calculate the internal volume, we'll then decide on a driver.

Anyway, I am going to make an MDF disk the size of the top of the inside of the spare wheel, as well as a fiberglass bowl in the spare wheel. My concern is how to bond the 2 together. I am familiar with wrapping it all in fleece and allowing the resin to bond the 2, but would that be necessary here? I am not sure I would want to bring the fleece around the top of the enclosure and not sure I'd be able to do it properly mounted to the bottom? Any ideas? Would you possibly recommend making maybe a 1/2" mdf ring about the same size as the outer diameter of the disk, fiberglassing to that, then mounting the MDF top to that?

Any help is greatly appreciated. I will definitely post pix.