I've made my props out of cardboard, so the best advice I could give on realism, is your paint job. It's less about the materials underneath, and more about the finish product that you see. If you're very good at weathering, or shading, it'll look great. Test out different paints, if money isn't an issue, automotive paint gives the best results, but you'll need proper equipment to use it. I just used acrylics.. because I'm poor.. haha
Proper ratio is important on the construction side of things. You want it to look good on you, not on someone else. So a little math may be needed... and look for as many reference pictures as possible to help see different angles & shapes.
As for attaching.. I haven't attempted it yet, but you'll probably need real leather, or something equally strong enough to support the weight of both props. How heavy you choose to make your sword an shield also matters on what you can use for support. Like if you choose to use wood, instead of cardboard, etc.