you may as well plan on building or buying. this sub will perform poorly for sure.
#1 your "enclosure" is not sealed in any fashion. Subwoofers in particular do not like air leaks ( with exception to an engineered port of specific size and length ) even the smallest of leaks can ruin a speakers performance.
#2 your "enclosure" will be way too big for this sub. in addition, the blanket you are using to "take up space" is actually working in the opposite of what you have planned. it is fooling the subwoofer into thinking it is in an even larger enclosure, similar to the use of polyfill, or an aperiodic membrane.
#3 Please anchor your sub and its enclosure to the car. I had a friend of mine years ago with a sub and box in his rear hatch, not anchored. someone hit him head-on, and while the air bag may have saved his life, the sub-box against the back of his head left him with brain damage for life.
there are a few posts in here of those who have built boxes that rest inside the spare tires, and some are very detailed in the building of the enclosures, as well. check them out, and maybe get a few ideas.
Heres one of them.
Round spare tire Subwoofer enclosure, Kenwood Excelon DNX690HD head unit install
Lets get that sub in there Slammin' but lets do it right, and safely.
