I made my son a Captain America costume for Halloween this year. I stenciled the stars and stripes on his shirt using screen print ink/paint and made his shield out of a frisbee and some more paint. He doesn't even know who Captain America is yet, so instead of focusing on the shield holding part, I just made elastic straps so he could wear it on his back like this.
And this picture is ridiculously cute. That's his favorite blanket in the picture. It goes everywhere with him - even on superhero adventures!