Matt,
That's an interesting design, I've never seen on like that before, (not that that means much).
I started with the commonly seen T cross section, and decided it wasn't stiff enough, and so added some bracing, but that didn't add any torsional stiffness, so I boxed it in. If I were doing it agin, I would just make the plywood box version. Mine is very heavy and is not readily movable. And in fact, when I build on it, I glue it to the floor so it doesn't move. (My floor is a "bit" uneven so any change in location of the feet of the beam would put it out of level.)
Keep the pics coming.
Dan