I tried to do a trash can helmet before I found out the material doesn't glue worth poodoo. But I did shape them before I gave it up.
I boiled the pieces, bent them, and then cooled them with tap water. I still have the pieces and they have retained their shape for almost a month now.
On a side note, I found a website that claims you can weld polypropylene (trash can plastic) with a soldering iron. I haven't had time to experiment with it yet.