You're probably better off making the armour plating from shaped plasticard built up in layers. They could be attached to a vest under the coat without the need for extra fixing straps and such. The extra guards and such can be molded and glued into shape.
Plasticard comes in a lot of different thicknesses and sizes. If you shop around I'm sure you'd be able to find a decent size to use. You can use something of a similar size and shape as your arm/shoulder whatever as a jig and clamp it down while the glue dries.
Ideally I'd suggest thin MDF and PVA glue, but MDF seems to be illegal everywhere except Australia.