I finally installed my subwoofer today after many months of going back and forth as to whether I would replace the GTI or not. Now, I can haz bass:
I had to come up with a mounting solution:
I used two 2.5" 1/4 X 20 bolts and some fender washers to make it happen!
Here is a pic with the sub mounted:
I used 4 machine screws with hurricane nuts and 4 drywall screws. The only reason I didn't use all hurricane nuts and machine screws was due to the fact that the hurricane nuts protruded out into the mounting hole for the sub.
I also had a change of heart regarding the GTI. After test driving a bunch of cars, I decided to stick with the GTI and modify it a little. It offers practicality that most other RWD coupes don't have and it does the job I need it to.