You neglected to state whether it has to be (readily?) removable, high or low precision, or if this is a one-off application or a production lot !
That said, a piece of wire, slightly longer than the inside diameter of the tube, held at its midpoint and forced into the desired location and oriented across the ID of the tubing. The inserted wire will be slightly bent and the bend will be free to pivot about the diametral line through its end points.
Or, with more holding force and potential for precision, a thin sheet metal washer similarly inserted. Washer may need to have radial slits spaced around its outer edge to reduce the insertion force and may be nearly impossible to remove without significant damage. On the principle of commercially available threaded inserts for common small sizes of tubing (e.g.,
You may have to make your own for larger sizes.
Or, a piece of wire, slightly shorter than the internal circumference of the tube, formed into a split ring whose outer diameter slightly exceeds the inner diameter of the tubing, compressed and forced into the tubing to hold by friction or into an internal groove for greater precision and retention force. Also commercially available.
A simpler way to expand the split sleeve is to tap a hole parallel to the axis of the tube where you plan to slit it with a taper tap, tapping only part way through the sleeve. Then slit the sleeve through the tapered hole. When you drive a set screw into the hole, it will expand the sleeve and lock it inside the tube.
Make a thick (.375-.5") washer about .002" smaller in diameter than the inside of the tube. Drill and tap the end for a 1" tapered pipe plug. Cut the washer almost the full length of the washer with a band saw through the center of the tapped hole. If you would like, you can rotate 90 deg and make another cut so that the face of the washer looks like a sliced pie. You have just made an ID collet that is tightened by inserting a pipe plug.
It's really so simple I'm surprised you had to ask.
Hold the tube in your right hand. Pick up the device with your left, and quickly insert it. The heat from your hand will expand the tube, and when you remove your hand the tube will shrink on the device. To remove it, just expand the tube with the heat from your hand.