Postage tubes are cardboard tubes posters and similar items are normally sent in. They are bit of a pain to carry normally since they are long enough you can't just stick them in your bag. Now on a scooter it is even more of an annoyance.
So I devised a method to carry such tubes easily on my back, using some stationary and shoulder bag strap. A picture of the this tube carrying device is shown below, and it should be obvious how to construct it.
It is attached to the tube thusly:
When both ends are attached, you can just sling it over your shoulder! :-)