I have made several ergonomic bolt knobs.
The AEA HP SS has the most brutal hammer spring and the smallest bolt knob I have seen.
I have also done that for the Crosman Icon, not because it really needs it, but it makes the pull slightly more comfortable.
The bolt knobs are made of thermoplastic, also known as moldable plastic or polymorph.
It comes in small white granules that you heat up (in hot water) and mold together.
It can be molded (like playdough), by hand, for a few minutes.
This is enough to make simpler shapes like a bolt knob.
It sticks to many things, and it slightly shrinks when it cools down.
This means that it will stay where you place it.
It is also extremely tough, and very hard to break.
The original color is white, but (using water based ink) the color can be changed to whatever you like.
I have attached photos of everything.