Minuscule fastener look

Occasionally I need to create the illusion of hex head fasteners smaller than the tiny nuts and bolts the hobby shop sells. Any ideas out there in model land?

Thank you

There are some folks who sell photo etched bolt heads/nuts. For even smaller than those, I dip the end of a toothpick in a puddle of paint the appropriate color and touch lightly to model surface. For instance, a medium gray looks like a flush rivet head on a natural metal finish. The model railroad venders have tiny styrene bolt heads and nuts smaller than any brass threaded fasteners. I try to keep a supply of those on hand.