So recently I pretty much "completed" probably one of my most dreamed of costumes: Marth from Super Smash Bros. I say from Smash and not Fire Emblem because I ended up doing one of the color swaps instead of the original. Anyway, probably one of the more difficult pieces was making the chestplate. Since the costume is done, I figure I'd share what I used and how I ended up using it to you all. Know that this is all pretty much from memory, and it is *very* picture-heavy.
Base: Football Pads
By far, probably the easiest piece to locate, but probably more expensive than other means (Mine, if memory served, cost me around $50 for the pads alone), youth football pads will serve you quite well in making a sturdy, durable chestplate base. This is important, especially since you will have a cape made of potentially two layers of fabric resting on the chestplate. The sizing is important: you'll want to get one that can come flush with your own shoulders, or one size larger at best (I went with the size larger, and it turned out incredible) for adequate coverage. You will also want to remove the actual padding, leaving nothing but the plastic. The ones I took in the pictures above have removable padding on all but what covers the shoulders; you will want to cut that off. You will also want to cut off the center pads between the shoulder and the main body (the ones that have "All-Star" on the pictures), as they are completely unnecessary.
Chest Covering: Funky Foam (Sorry for the blurry picture... ; ; )
This stuff is a godsend: a foam sheet that feels like felt, very flexible, and easily cuttable. It makes the perfect front piece for the chestplate. Now, while I did my costume in white...
...there are many more colors of this stuff to choose from, so any variety of color palettes are available. My chestplate front itself was made with one single piece of Funky Foam, cut and designed to appear like the front of Marth's chestplate (reference pictures or Melee/Brawl itself will do quite nicely), and was hot glued to the front of the pads to look like such.
when you hot glue this to the chestplate, leave enough room for you to reach underneath the sheet to hook the pads to yourself! The pads hook underneath your arms to the front in little slots. You block them, or a way to get to them without mangling your front, you're doing it wrong. =X
Shoulder Paint: Rust-Oleum Specialty Plastic Spray Paint
You'll specifically need something to cover up the unsightly silver plastic on the shoulder, and nothing screams out loudly like spray paint. But most spray paint won't work on projects like this (trust me, I found this out the hard way), so you will either need to sand down the padding or get a special type of spray paint. This is that kind of spray paint. It binds directly to plastic, without any sort of primer or sanding, and it leaves a very good sheen to the painted area. It doesn't melt the Funky Foam, so if you forget to apply this until *after* you attach the chestpiece to the front (like I did), you can still spray over the shoulders without fear of losing the foam sheet.
One thing to note can be spotted in this picture:
The shoulderpads aren't even truly noticable, right? So why paint over them? Simple question with a simple answer: it all depends on how you get the cape to hang. You will *need* to cover both shoulder pads with the paint almost as an insurance policy, because if your cape hangs differently, more of the shoulders will be visible.
Edge Trim: Shiny Gold Ribbon
No two ways about it, for the trim around the chestplate, shiny ribbon is the way to go. I just happened to need gold ribbon, but depending on the color swap you choose, the color may vary. Regardless, you will want to cover every edge along the front and the shoulders with this; hot glue is your friend for this. It's also recommended that you have a second person help out for this, as you will be having to deal with interesting cuts, folds, and pieces in order to get the look right. Just note: you'll want this stuff by the spool. Trying to figure out yardage you need may be too complicated, and you never know when you may mess up.
And there you have it, the list of materials, why they're used, and how to use them in one (hopefully) informative post. With any luck, your chestplate will look as good (prolly better =D) as mine:
Feel free to ask any questions, and I shall try to answer them to the best of my abilities.