Presuming you have some tools on hand ahead of time and depending on how you decide to embellish it, this is a really cheap but effective method for making a costume piece. I'm assuming you have at least the basics like scissors and craft glue when I say under $5, probably.
I've been seeing tutorials everywhere about making costume armor out of craft foam. The results usually look awesome, so it got me thinking that maybe it would be a good material to make a simple cyborg-like prosthetic. The result was a lightweight, semi-flexible piece which was form-fit to my face.
And here's how you can make one too.
Step 1: Materials Needed
For a basic piece, you need:
-1 sheet of craft foam - the thinnest available
-White craft glue, Modge Podge - a thick, water soluble adhesive
(if you don't have this already you can find it for under $2.00, get the smallest as you don't need much)
-Scissors, craft knife - something to cut the foam with
(You probably already have something that will work)
-Something that smudges that you can draw on your skin with - cheap eyeliner, dark chalk, charcoal, you could probably get away with chocolate syrup to be honest
(again, you probably already have something)
-Liquid latex, spirit gum - some adhesive to attach the prosthetic in the end
(probably the most expensive part of this, you can get tiny single-use applications of either around Halloween for generally less than $3.50)
-Paint of your choice and brushes
(if you don't already have this you can get cheap acrylic paint for $1.00 at craft or dollar stores)
(you must already have this)
Things that may help you along the way are:
A glue gun
Circular stencils, or objects to trace around
And extras may include:
Small metal bits for embelishment
Face Paint, to tie it all together
Lenses for your eye - from sunglasses, old goggles, a round piece of clear plastic...
Step 2: Creating a Pattern
This is where the smudgy drawing tool comes in. It's important to think of where the seams between pieces of foam are going to go, because they determine how smoothly the piece will sit on your face. I wanted a sleek, almost skull like fit with a big glowing goggle-eye-lens-thing, so I followed the natural contours of my face. If you wanted a bulkier or more boxy fit, you wouldn't have to pay as much attention to the face underneath.
Using your smudgy drawing tool (I used some old eyeliner) draw directly on your face where you want the seams to go. Mark the edges of the piece and trace along any hairlines you encounter. It's a good idea to decide what you want to do with the eye now as well. If you want it to stick out cylindrically you're going to have to draft another piece, if it's just going to be a socket you can refine the shape later.
Once all the lines are in place take a piece of paper and press it smoothly along one plane, rubbing down along the lines you created. Tissue or thin tracing paper works better, but it can be done with regular paper. When you pull it off, there should be a residual outline of the shape you want to create. Do this for each plane you drew, cut the pieces out, and you've got yourself a pattern.
It helps if you keep notes on the pattern about which seams match up where. This helps prevent confusion later.
Step 3: Cut and Glue Foam
Glue the foam bits together according to your pattern. Only glue the basic form (the part that will actually be touching your face) together - any fancy eye bits or embellishments should be added on after the piece is sealed and fitted. At this stage you should have an annoyingly floppy piece of something that may vaguely resemble a face.
Again, I didn't take in-progress pictures, so anyone thinking that these images are intense closeups of scraps and leftovers...is absolutely right.
Step 4: Sealing and Fitting
Attach the piece as best you can to your face. If you're having difficulty you can do the following in sections while holding the piece on in the position that you want it to attach. Until the foam is firmer attaching it can be challenging.
Paint a layer of white craft glue over the surface of the foam. This first layer can be sort of thick, as most of it will be absorbed by the foam. Let the layer dry, then paint another, this time thinner. The idea is that the glue is soaking into the foam, thus both strengthening it in the shape that it fits and providing a surface that will be easier to paint to look like metal. I would do three layers minimum, though any more than five would probably be excessive.
Let it dry completely before removing it from your face. Use whatever remover your adhesive came with as necessary. It may crack a little coming off, but it's not a big problem and a brush of glue should disguise the crack and seal it back up. You should be able to pick up the foam without it flopping at all, and it should mimic the contours of your face.
* A side note - though sealing the underside of the foam would probably make it adhere to your face better, I would advise against doing it with the craft glue. As you sweat, the water based glue is going to liquefy and turn into a gummy mess under the foam. However, you can coat it with liquid latex.
Step 5: Paint, Embellish, Detail...
I had some EL wire hanging around, so I poked holes in the side of the eye-goggle-thing and ran it under the lens to make it light up. No, I could not see out of it. In retrospect, I could have moved the eye over slightly and disguised an eye-hole under a grate or something. Or I could have left the whole thing open and worn costume contacts. But EL wire is what I had and walking around in platform boots with only one eye was a fun challenge.
Then comes painting. This can make or break the piece. There are plenty of metallic painting techniques out there if you are uncomfortable with your skills or have never painted fake metal before. If you aren't sure what direction to take, I suggest looking up some tutorials on Youtube. The slightly weathered silver that I painted mine was done by a combination of dry brushing and rubbing silver paint over a black base with a rag. I then went over it with black paint in the end to help make the details pop.
Step 6: Finishing Touches
Hope this helps serve as some inspiration. If you have any questions or feedback let me know!