I took the plastic underseat storage container I had originally put in the truck and I put it back in the bed after I purchased the cab bag for under the rear seat instead. Something similar to this
I've got a cargo bar, and I place that across the bed near the tailgate and now put that plastic bin up against it to keep it from sliding around. Works great for storing stuff when shopping and keeps it close to the back to make it easy to reach. I'll add a picture in a bit, just have to go out and take one.

Edit to add pic. You can see the cargo bar peeking out the left side behind the bin.