Chell's Portal 2 portal gun
I think the whole Chell cosplay was just an excuse to build the portal gun. I actually have very fond memories of that time. I was preparing for the biggest party of my life, I had a lot of fun crafting the main body of the gun and most importantly, I ordered the materials I needed very early on, so I had plenty of time. Everything was on schedule, every little detail worked perfectly, I managed to finish it ahead of schedule, clean the "workspace" and play around with both the boots and the gun the night before the convention (normally I'd stay up till 5am to finish crafting, and then sleep for 3-4 hours before the con).
So! I started with the shells. I cut, glued, warmed up and shaped EVA foam (10mm). I ended up gluing both pieces to a plastic bottle for the duration of the crafting because they kept losing their shape and the paint kept cracking. Lots and lots of acrylic putty, lots of sanding, and more putty. Lots of downtime too.
When the texture and smoothness of the shells was to my liking, I painted and varnished them. I had to varnish them again, a few days before the convention, because there were cracks on the varnish, from the high temperatures of the room.
I made a long tube (out of EVA foam), decorated, carved, glued, primed, sanded, and painted it. I found a thin clear tube, which I used for the main core.
I ordered EL (electroluminescent) wire. A blue and an orange one. Unfortunately the orange wire turned out to be faulty. After a few days the switch (or battery pack thingy) broke and it wouldn't work. I settled for just blue.
Pretty happy with it, I was playing around the house the night before the convention.