If it is metal tubing you might be able to expand a one of item by
freezing water inside or using a mandrel and a hammer like a snarling
iron on the inside. A bolt with the head filed round would be the tool
to make. Pound on the shaft of the bolt to transfer the force to the
head inside the tube working around the inside. Might involve a bit of
annealing too.
If the tubing is some kind of rubber or plastic try dipping in in
boiling water and opening a pair of needlenose pliers inside.