Tip, try to duplicate the metal discs layer, then set the brightness of the duclipated layer to zero, put this solid black image beneath the previous layer and create 3 copies of these.
You now have four solid black figures of the primary layer, you will then move each (1 or two pixels, ctrl + up/down/left/right arrow key) Combine these four solid black layers. This will somewhat create a shadow of dark outline of your image. I usually reduce its opacity between 30-75%
This is how I usually create hard shadows in 2D texture making.
I something create two sets of shadow layers (what I casually call them)
One is a few pixels moved (Like 2-3 times in each direction) This has a lower opacity, maybe 30%
And the other one is moved only 1 pixel in each direction (Just like in visual arts, the shadow tends to weaken as it goes farther from the object) So this layer has higher opacity since it is the one closes to the primary layer (or the layout/shape layer, what I call it)
Just my 2 centavos in texture making