The space is really just to make it easier for people (mostly my son and friends) to climb back into the boat from the swim platform ladder to the right, rather than having to climb through the splash well right where the fuel line goes. Also, while I know it serves a purpose, I just hated looking at that ugly splash well back there. As to the structure of it, here are some more photos. The two long planks are bolted through the splash well itself. So are the shorter ones, but they also have the 2x2 braces attached from underneath connecting them to the long planks. I may yet build some kind of brace from the 2x2's to the bottom of the splash well, but we'll see how it goes to start. The p;lanks are cedar decking with three coats of spar varnish. All the holes were filled with marine sealant.
The one thing I was a bit concerned about was the weight, but the boat is rated for a 140 (later models a 150) and we only have a 115, so the additional 40lbs of wood back there is less than if I maxed out the motor.