Revisiting subwoofer ideas

Not looking for anything crazy... My OCC has a Kenwood head unit and Kenwood speakers in the stock locations. Sounds great for mids and highs. Just nothing on the low end. Looking for ideas/ solutions to install a small subwoofer either under a seat or possibly in one of the rear compartment areas.... I am not looking for bass you can hear outside of the car at red lights nor really any ground shaking.... these cars are 30 year old GM plastic inside so it would rattle like a mofo. Just something that sounds clean and more in line with the Bose in my pickup or Harmon Kardon in the Palisade.... nothing that vibrates your nuts off but something thats just enjoyable to listen to when cruising. I've searched the forum and found some posts, but they are 10 years old or so...just wondering if any of the newer self contained enclosed types of subs have had any luck. not looking to fill the cargo area or the footwell for the 3rd row... something out of sight, out of mind