I drove my ship into the t shaped collision mesh and it stopped. Shots went through the ground.You said the latest version collided for you? I downloaded it and tried it and it did not collide for me.
It showed up in debug.collisions ok.
Its not a terribly good point in development to be trouble shooting collision meshes.
But anyway, it needs to cover the base plate, it can also have separate pieces, so you could add 2 boxes for the turret pieces