I set my Domino T-Lock Systainer box on top and locked it with the T-Lock. When you go to move it or pick it up it just tilts and unlocks. I acts like the back is not locking in.
The hose and cord on the Mini and Midi will cause the T-loc to not fully engage in the back catch. Work to make sure the cord and hose are down low enough and you can get the T-loc to latch. I was dealing with this today. I put the hose in first and then coil the cord into the empty space and press it down before putting the Systainer on top. I believe the new hoses that Festool is now shipping might make this easier to deal with.