Hello again. Thanks so much to everyone who replied to
my previous post. I didn't have time to reply to everyone since all of my free time was spent working on the costume, but I did read them all and got some great ideas from you guys.
I originally didn't have any plans for Halloween, but I got invited out at the very last minute, which meant I had very limited time to come up with an idea for a costume and put it together. From Friday night to Sunday afternoon I spent almost all my time shopping for pieces and then assembling the costume.
I lucked out and found this roomy felt hat that I cut a hole into and then hot-glued the blue lid from the inside. I then attached the light to a spare piece of fabric and velcroed it underneath the lid so that I could open it and turn the light on and off. It was very sturdy and lasted the entire night without a hitch. And it was a total hit! All night people came up to me to ask where I got it and were really impressed when I told them that I made it. Which baffled me a little because it was so last minute that I felt it was glaring how thrown together it was. Apparently not. Also a lot of people just came up and touched it, which was really weird, but ok.
I was so lucky to find this dress on my first night out shopping. It was cheap and the only one in my size and just perfect for the TARDIS, imo. I thought I would cry tears of joy when I tried it on in the fitting room. I bought some fabric paint and hastily applied the design, which also (to my complete astonishment) surprised and impressed a lot of people.
Overall I am happy with the result, though I am a bit of a perfectionist and regret not having more time to work on it.