So... now I have been to the TTT premÃ©re in my Gandalf hat. Thanks to everyone who helped me with tips on how to stiffen my hat brim. I found out that Aileens does consist mostly of diluted PVA glue, so I used glue. I smeared on lots of it between the two pieces of my brim. It took a few iterations of wetting and shaping to get it right. The brim got really stiff, but has a soft surface. A picture is here. Yes, I am wearing my Jedi robe. I didn't have time to find/make a nice fake beard/hair, so I dyed my existing one, and I did forget to bring my scarf.