Hello all.
Did a Combine Civil Protection costume a few years ago.
I went with one of the actual Soviet gasmasks and just modified it. It's far from perfect, but the final product looks decent, I think.
I cut away the back of the mask and used a cheap plastic German helmet replica for the back of the metal "skull cap" portion (I melted the lines and details into it, then painted multiple coats of grey and silver and watered down black to fill in the details). I replaced the clear lenses with oval shaped cutouts from a pair of mirrored sunglasses. I used plastic clasps that I glued and sewed to both the rubber mask and the plastic skull cap sections to hold it all together. In retrospect it was unnecessary to have them so they could be undone; since it's rubber you can still fit your head into the mask without taking it apart.
The brown neck/collar part was sewn using real leather, and brass grommets. The grommets were nice because they actually allow you to hear while wearing the mask and line up almost perfectly with your ears. There was a fabric collar beneath that with a single fasten, which helped hide the seams and wires.
Used a small cat food tin to make the lower gas mask filter larger, with large industrial bolts and rubber "feet" glued on top for both.
The "headphones" I made by cutting the intricate pieces out of aluminum pop cans and half of a glow in the dark rubber ball (I never got the antenna parts done, so those are missing).
I bought an Optimus Prime voice changer helmet and stripped out the electronics for the Combine voice, and I wired the speaker from the Optimus Prime helmet into the front "snout" using a small grill to space it out so that it wasn't muffled by the industrial bolt on the front. Distorted feedback was a major problem, because the mic was only a few inches away from the speaker, I tried to fix that by putting a bunch of foam between the two to isolate things, but if I were to wear the costume again I'd probably try replacing the mic with a military throat mic or something, or figure out a better way to isolate the them.
The vest was custom made, sewn from rubberized black fabric material and the individual sections padded with foam, and coat hanger to give shape the "bomb disposal" neck part.
The under jacket was an army surplus mandarin collar coat (chosen for its length below the vest and lack of lower pockets, with the upper ones concealed by the vest), the white shoulder pads were vinyl.
Belts were made using regular webbing belt material with custom metal non-functional buckles over top of functional plastic clips.
Walkie talkie was the shell of an off the shelf walkie talkie with an iPod inside plugged into the speaker playing Combine radio dispatch chatter (too quietly to be heard at the noisy con). The smaller walkie talkie/locator thing on the opposite side of the belt was missing from my costume).
Pants were black army, with cargo pockets removed and light reflective strips sewn on (matching light reflective strips on the upper arms and for the square in the center of the vest).
Boots were off the shelf Soviet military surplus (which seem like a fair guess at what inspired Valve, given the Eastern European setting of the game and the Soviet gas mask source).
It was a very uncomfortable costume and I could only wear the mask for 5 minutes at a time, largely because of the added foam I used to try to isolate the voice changer. If I were to wear it again I'd try to add some fans/ice packs. At least there's plenty of spare pockets for ice packs..
Sorry, not the best photo of the full costume; some parts of the mask weren't adjusted properly:
).
