Use a dremel to make the hole in the top plate

Dremel also removes material on the back side of the plate so that the ball is close to the surface.

The ball's ribbon cable goes through careful cuts in the membranes of the keyboard.

You can swap a hub's shorter USB cable for the long one the keyboard comes with.

ot glue or epoxy attach the USB hub to make a permanent foot. (You'll have to add something to the other side as well)

You can run most of the trackball's USB wire inside they keyboard above the F-keys.