Good luck! Let me know how it goes and what materials you try, what works, what doesn't. I would be interested in putting together a set too for double checking samples.
My understanding of the system would be that an 18 screen would be 18/64th's of and inch, a 19 screen would be 19/64th, 20 would be 20/64ths...and so on.
Screens can be purchased here (and other places), although I do not know how costs would compare to building your own. They are a precision tool (usually certified for accuracy by ASTM), so unless you have the proper tools, knowledge, and ability to build a precision screen, buying would probably be a better option. I would think one would want screens a few 64th's smaller as well to see if beans are undersized.