I am using an aluminum tee. It was purchased as a 1 1/2" wide and tall piece. I simply trimmed (the bottom, horizontal section) to 1 3/16" wide on my table saw with an appropriate blade and safety devices. It fits perfectly on the underside of the gantry using the same bolt-through treatment as the flat bar. I will post some pics when I get a chance. I believe the aluminum tee is more rigid (and lighter) than a flat bar. The bottom of the tee cradles the makerslide and makes it virtually impossible to sag--it holds itself up.
If you are uncomfortable cutting aluminum, or are not equipped to do so, a local machine shop could do this for you for an affordable price.
I had a steel flat bar in place but decided to try the tee due to the observation I had of the gantry "bouncing" a bit during fast direction changes. There was no longer any noticeable flex, but rather a certain inertia present when the Y axis changed direction. Fastening the whole frame to the table may have eliminated this, but my thinking was that over time the extra weight of the steel may wear on the motors and belts. The aluminum tee is half the weight and is, in my opinion, a better solution. Sag, bow, and twist are gone from the gantry--as well as the inertia bounce. I'm not an expert on the physics by any means, but my eyeballs and test cuts tell me the solution I have implemented works the best for my situation.