Ewe, if there's anything I hate, it's straight slot screw heads
But what you could do to hold them is make a split fixture block. A simple way to do this is to get an extra deep nut (a coupling nut) and saw it down one side with a hand hacksaw. You can then collapse the nut in a vise type fixture and grip your blanks without marring the threads.
Or, you could get fancy, and make a fixture to do the same thing, starting with two pieces of stock, shimmed a few thousandths apart. Drill and tap holes right on the edge, forming half holes in each side of the block. Remove the shim, and then install your blanks in the fixture and reclamp.
BTW, PM me if you are in need of a cross slide vise for this project.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)