The sheet of HDPE sounds good. There is a neat guide on here for cutting HDPE (https://www.fightingrobots.co.uk/threads/14279-a-basic-guide-on-how-to-work-with-hdpe?p=474927) which I would definitely recommend reading.

The Sportsman category at RoboDojo is definitely a good laugh, worth going to. I am nearly never able to attend them with it being on a Sunday.