Today only. Amazon with Drop Stop LLC has a 2-Pack of Drop Stop The Original Patented Car Seat Gap Filler (As Seen on Shark Tank)  for $18 with free shipping on $25+ or with Prime.

The Drop Stop attaches to your seat belt catch and moves with the seat; expands to fill whatever size gap you have and catches dropped change, phones, etc. Also comes with an LED credit-card light and cell phone grip pad.