Just to add to Helena’s suggestion, ABS pipe is considered stable and comes up to a 6″ diameter. For a larger diameter, you can use the large cardboard tubes used to pour cement footings but seal them first with Marvelseal or make a barrier as per CCI Note #1-9 Low-Cost Plastic/Aluminum Barrier Foil http://www.cci-icc.gc.ca/publications/notes/1-9-eng.aspx .
