Since then I've added quite a bit more to it. I turned to Ana White for her free plans. The plans for the base sections can be found here, and the plans for the cubby portion can be found here. I made a few modifications since I wanted the storage unit to be built-in around our 8-inch ledge and wanted the unit to look like this:
(Pottery Barn Cameron Creativity Storage System)
Warning: Mine doesn't look that pretty...yet.
Once it was all built I was unhappy with the edges not being completely smooth (since I used MDF) and I wanted it to look seamless where the units met up. You can see what I'm talking once you see the photo below:
So I bought a bunch of trim to cover it all up. That made it look so much better!
Here it is all painted. FINALLY! It's been a long time coming. I obviously need to still find/make some bins to make it look all cute, but for now all the toys have a place, and I love that.