Portal 2 is an awesome game and the turrets were one of my favorite parts (even though they shoot at you). I wanted to make one and I had the parts, so I did.
Black twisty- ties
3V button battery
I started with a Styrofoam egg that I found from another project. I bought mine at Micheal's about two years ago (I never used them since).
First, sand the sides so the egg is more turret shaped.
Then, use a box cutter or another thin bladed tool to cut about 3/4 inch groove in the back.
Next, cut three black twisty-ties to about 3 inches. These will be the legs. Make the third one a bit longer since it will be the back leg. Bend the ties into the leg shapes and stick them into the body.
Stick the LED through the front of the body so that the leads are visible in the back groove.
Now for the leg plates. Draw the design on a piece of paper and cut them out. Stick them to the legs with the Glue dots or another form of adhesion.
Finally, stick the 3V battery into the groove with the correct leads touching (the long lead on the LED is positive) and watch your turret's eye light up! Now your sentry turret can stand guard over whatever you choose.