For something so trivial, low on detail and small like this just cast some from real screws using ghetto casting methods... Total cost a few dollars...
For the resin, get some two part epoxy, 5 or 10 minute epoxy will work fine, 1 hour epoxy will work as well, just takes longer...
Get some modeling clay, either WED or oil/wax based for the mold... Heck Play-Doh will work for this project...
Step one, roll the clay out on the table
Step two, press real screw heads into it, pull out
Step three, repeat step two until you have several molds
Step four, mix some resin on a paper plate with toothpicks and drip (small drips will reduce entrapped air) it into the molds
Presto dozens of screw heads on the cheap...
If you want real metal I don't think you will find anything as economical, realistic or practical as cutting down real screws...