Of course, concrete! Why didn’t we think of it before? It’s as easy as creating a form out of a something small, in the shape you desire for your knob. Then fill it, and let the concrete set for a few minutes. Insert the head of a bolt, and let the concrete dry. Once removed from the mold, sand the knob smooth with silicon carbide wet/dry paper and rub the surface with talcum powder to create a shine.