In this case, outside does not convey what you are trying to say. Outside here means that it is not inside. It could be anywhere outside: across the room, across the country, etc. Something like
The hollow tube holds the solid rod.
which would indicate that the rod is stored inside the tube, or
The hollow tube contains the solid rod.
which would indicate that the rod is inside the tube.
Your use of "located" is offputting to me - for your first sentence, I would usually say
The solid rod is inside the hollow tube.
in casual speech. However, if this is from technical documentation (it sounds like it is) I would keep it the way you have it.