There seems to be room under the front seats for some type of storage tray or box or whatever. Has anyone does anything with this space?

 

As long as you don't have a HK stereo.
HK has subs under the seats.

 

Standard audio has subs too. If that's what you want to call em LOL

I use smaller vacuum storage bags to keep things in under there. A box might be hard to get in and out depending on size, but the bags work well because they are pliable.