Prefab boxes and being loud, normally do not go together. My formula for building a loud system is as follows...
Decide what you going after, SPL, SQ or a little of both.
Choose subwoofer(s) that best fit your above choice(compare there TS specs to determine compatibility)
Determine which enclosure would best meet your end goal. For spl, ported, 4th, 6th, 8th order bp or for all around sql performance a t line box works well.
Your vehicle is part of the enclosure, build it(your enclosure) as such.
Build the right box for your car and the bass will come no matter what sub you use.
Normally tiny enclosures will produce tiny results.
Keep panel resonance down to a minimum to achieve a better tl score.
My last spl build I used 500lbs. of concrete and 1000lbs. of mdf, 2x4's, screws, glue etc.