Hmmm, why do you have to attach a hollow tube to the end of the output shaft?
Why not just stick the output shaft OUT of the hole in the box, then attach the "spindle" to that?
It looks like the motor has a D shaped shaft on it, so all you need is a pulley that has the same size and shape hole as the output shaft of the motor. Shouldn't be too hard to find. (Or a drive pulley with a set screw)
Oh, then you could simply drill a hole in part of the pulley, thread the wire/whatever through it, tie it off(securely), and then it'll be your "spindle".
It sounds like you're making a giant grandfather clock. That's a cool idea if it is what you're doing.