Here's my costume "Stabbed in the Back" I hope this will help you with faking an object coming into or going out of your torso. I have a big woodshop so if you dont have a all the tools I have for wood working you might opt to use an easier to cut material like insulation foam board, or art board. It only has to last one night.
Step 1: Gather Materials
I used a piece of cherry for the blade, 1/4 - 5'16th thick and large enough to cut the design out. I used aluminium HVAC tape for the fake metal surface, you could go with silver spray paint or maybe glue aluminium foil onto the surface. The handle I wrapped in black tape. The belt I used is one of those preppy belts from the eighties that uses a clasp instead of a buckle and holes. The reason you need this type of belt is so you can tighten it as tight as possible. I attached the blades with a wide washer and drywall screws. The back part wanted to flop down, so I added a piece of metal that extends under the rig, giving it some support.
Step 2: Draw You Knife!
I wanted a cartoon appearance so I exaggerated the size of the blade. I drew it out fast and chopped it on the miter saw at an angle so the blade handle would be up a little. Next I cut the rough shape on my bandsaw, you could use a jig saw or a hand saw of course.
Step 3: Sharpen the Blade
I have a bunch of specialized wood working tools, so use what tools/ knowledge you have for this step, or dont do this step if you feel its dangerous. I used a draw knife for roughing out the blade, and cleaned up with a spokeshave. It looked good enough as far as I am concerned.
Step 4: Add a Guard
I think this knife part is called a guard or its a quilon? I added it for realism, I had some scrap wood from sawing boards on the table saw. Chop to length and glue with clamps.
Step 5: Fidget With the Connections
I thought it was a good time to test out the design before adding the finishing touches. I predrilled holes and attached the wood to the belt with drywall screws through wide washers to make it more stable, It worked well. The back part of the blade would droop so I added a flat metal strap below to support it. You can see it in the first pic extending under the bottom of the handle of the knife. One note, It took me a while to adjust the blade parts to make them line up, hopefully you have somebody to help you, I had to do it in a mirror.
Step 6: Finish the Blade
I added the aluminum HVAC tape and rubbed it out flat with my fingers, later I burnished it with some steel wool. I added strips to the bone saw on the top ridge of the blade, I think it worked okay.
Step 7: Wrap the Handle
I used some matte black gaffers tape to give it a tactical look, I added more aluminium tape to the end of the handle. I wanted people in a crowded bar to be able to see the handle!
Step 8: Destroy a Shirt
Pretty easy, mark your holes and cut it open where the blade enters and exits. I used red spray paint, I held the trigger down until the paint ran down, giving it a bleeding effect. If I use this in a small film, I'll add fake blood. Since I'm using it for Halloween, I used a paint that will dry so I'm not a bloody mess all night. The third pic shows a bag I cut a hole in to keep the surrounding area from getting sprayed. One note- the shirt I chose would rip open easily so I used some spray adhesive and attached a piece of t shirt material above and below the cut on the inside of the shirt to keep it from ripping all the way open. Sorry for the run on sentence.
Step 9: Bloody Up Your Knife
I used red finger nail polish, it stays sort of wet looking.I put more on the tip since it has passed through my chest.
Step 10: Test It Out
When you've got all the parts together, give it a test run. I walked around a bit to make sure the harness didn't begin sliding down. I'll be wearing this out to several bars, so I don't want it coming loose. Well that's it, Happy Halloween! One note (after wearing it on Halloween) be careful to not hug on old friend like i did, they were not happy. haha, I totally forgot I had a large pointy thing coming out of my chest. Maybe put a rubber tip on it?