In case anyone is interested in getting a closer match to the proper thickness for the flat bar stiffening mod (I couldn’t find this as a t-bar):
These guys have very decent prices on 4mm * (5-50)mm * 6 foot aluminum flat bars. I bought one for $25 (including shipping). Being slightly smaller than the ~4.1mm optimal spacing means it’s easy to add something to shore up the difference.
If anyone finds metric aluminum t-bars in The States, 4mm thick, with short wings (so we don’t need to trim) please let me know.