Purchasing D-shaft is not legal, since the flat makes it a custom machined profile, and only round, square and rectangular profile metal bar material is legal.
You have to buy hardened round shaft and grind (Dremel) the needed flats, for at least the lengths where the screws grip, if not the full length.
We design to avoid, as much as possible, applying any torque through the TETRIX shafts, and to always have both ends of shafts supported when bending forces are present. Bolt the gears together for compound gearing, so the shaft isn't transferring the torque.
When we do transfer torque through TETRIX axles, we try to always double hub them, on both sides of gears or sprockets, so that one single set screw loosening cannot kill the mechanism.
You can use long, Grade 8 10-32 SH cap crews for free spinning shafts. On last years bot we did this for our VEX wheeled, chain driven mecanum drive. The smooth zone of bolt was polished to fit the TETRIX sleeve bearings. Worked extremely well, but length is limited.
-Dick Ledford