A lot will be determined by the amount of space you are willing to give up. But a single sub in a small box properly installed will sound good. If you are going to DIY then you can start with one of these
https://www.soundoffaudio.com/item.cfm/id/2105 or
https://www.soundoffaudio.com/item.cfm/id/2206 I'd recommend fiberglassing the inside of either before you install it. I have this same brand and its pretty flimsy as it comes from them. I pulled the carpet and fiberglassed the thing inside and out. A little bit of effort but it paid off in the end. They are a good start for a DIY setup. Still maintains some storage in the rear. You can also go with Mr Marv's boxes, very nice but a little more $$. I can vouch for the Pioneer Premier line of shallow mount subs. Not too much $$. I have two 12's and anyone who's sat in my truck recently can probably attest to how they sound. But driven by a good quality amp should do the trick. I have a single Alpine PDX 1.600 powering my subs, its really about 750 watts into them and they get loud enough to hurt your ears but yet they are very clean.. very tight.