Behind the seat subwoofer

So i have been debating on getting a rear sub behind the back seat. I have an aftermarket Kenwood double din and Hertz speakers already but i’m looking for some more bass but nothing crazy and i’m not looking to spend a lot of money. Maybe under $300 or so.
My buddy can maybe build me a single sub box for behind the rear seat. Looking to run either an 8 or 10 inch and an amp. I know not much about car audio so i was looking to see what y’all recommend. I listen to rock and metal music mainly and am looking for more bass but like i said nothing crazy. Just enough to satisfy what i want.

 

Go to Crutchfield.com look up their shallow mount subs then choose an amp.

I run a JL Stealthbox, JL 10TW3 with a JL XD600, it sounds great. Had that running with stock head unit and sounded great even better with my upgraded Alpine ILX-259