My writing group and I decided that we simply MUST dress up for the Harry Potter book and movie releases this summer. So we had a couple of marathon sewing sessions, and made robes! Four of us, four houses... one of each!

Here's our group together. (The robes don't really hang as strangely as mine - I'm the one on the far left - are in this picture. We used elastic as the fastener, and that's just not working out well. Skirt hooks are on the docket before the book release.)
Our wands, which we made from dowels and sculpey:

And my service dog's very complex and complicated costume:

And my service dog playing the part of a Hufflepuff unfortunately transformed into a poodle by a mischievous Slytherin during Transfiguration. The scarf was knit by our group's 'puff.

Other pictures from the event, including other folks' costumes, may be seen
here.
I'll try to get some better, full-length pictures at the book release to edit into this post.
Other things that I will get better pictures of at the book release:
our House Unity pins and Dumbledore's Army pins, and my awesome Luna Lovegood messenger bag which turned into AwesomeSwapHaven07 with pins from one swap, an owl from another, and the bag itself from another.
Edited to add:
We used
Simplicity 9887, with the modifications mentioned
on costumebeginner.com, and also we winged it to make a full lining. It was a pretty forgiving pattern, and really easy to put together, once we figured out how to attach the lining so that the sleeves didn't do something funky.