Never saw anyone else try it, so I figured it out myself
Basically took this off ebay (not a hot toys one, just a diecast trinket in the same scale)
Ground off the star, carved in the eagle
Filled it and sanded it and painted it one thousands times
Molded it
Cast them
Added some detail, the fist has a magnet embedded inside so it doesn't show on the back of the hand, the shield has a magnet glued on the inside. Not perfect but it makes it movable and removable
Its an energy shield, so it has to glow too (europium glow powder and gel gloss medium painted into the lines)
Took easily about 20 casts to get the process right, turns out thinner is vastly more challenging, also I didn't know what I was doing. Ended up with a vacuum chamber to get the mold material done without bubbles, and a pressure pot for casting the resin. And 7000 grit sandpaper.
Shield bros