Under the back seat is the best place for additional storage. There are options to purchase. I opted to build a custom one out of 1/4 inch plywood, painted it with bedliner paint, lined the bottom with black outdoor carpet. Created the template with cardboard before cutting the wood and got exactly what I needed.
Got any pics of this project?
I have a 19 custom. There is replacement (GM?Chevy) center (middle seat) that has a console. Looks just like the factory one but opens. I was shocked too when it did not open. A word of caution if you decide to replace that middle seat(and spend the $$$) it is a bitch to unscrew the floor bolts (unless you have the right tools). I found a slightly used one on ebay for $325 shipped earlier this year(bought TB in March. Honestly it is not a lot of storage but enough for me. Let me know if you would like pics. Good luck. The other storage as others mentioned is under the rear seat. I have seen anywhere from $100 up depending on material (plastic/ metal). I again lucked out locally and got a Tuffy box with 2 doors and locks for $225 new from a guy that bought a GMC that had it thrown in the back seat. Those go for $375 and up. I had to modify the brackets but it fits perfectly.
I agree on the swing cases. Had them on the last 2 trucks. They're great with plenty of storage
