The problem is "fairly loud", that's extremely subjective. At 20yrs old my perspective of fairly loud was much higher volume or harder hitting than it is almost 30yrs later. I plan to make a custom sealed box and install a 10" sub with 400watts. I expect it to be far more than I need/want. I do not want to advertise what's in my vehicle I've had plenty of stereos stolen in years past. I just want good, clean, full sound that stays inside my vehicle.
Here's an article on doing this:
https://www.expeditionforum.com/threads/full-stereo-system-build-pics.11014/
You could more than likely put a 12" sub in the factory location if you can find one that will work in a 1ct ft box or less. To get 1cu ft you'll have to use every bit of available space in the factory location though.
If you want something seriously loud and hard hitting, you wanna be hear down the street, you'll need to use multiple subs and/or a large box outside the factory location with a big amp I think.
