I have a '21 F150 regular cab & have been looking for a storage option behind the center seat. Found a crate from the Container Store measuring 15-7/8 × 9-3/4 × 10-1/4. In grey also which is close to the grey interior. It fits tight & won't move around. It is fairly deep at just over 10". At $16 it is a pretty inexpensive add. If interested just go the Container Store & search for "small modular stacking crate".

 

Link here.

https://www.containerstore.com/s?sou...stacking+crate

 

If you're not opposed to making permanent modifications, you can pretty easily drill through the floor there to bolt something down. I've drilled through the floor there to mount boxes and other items on older models without issue. (Just visually check before you drill.)

 

Also, these are pretty awesome (and lockable/easily removable when you need full bed space).

Swing Case Truck Tool Box - a Truck Accessory from Undercover