I jigged the spars level on the open bench, chocked them in place, and bore holes for the sheave slots. A Forstner bit did quick work of it.
A chisel and utility knife roughed the edges to shape. Filing completed the needed slot width.
I test fit the sheave with a pan head machine screw. I'll replace with a counter sink screw and finish washer later. he machine screw may be tight enough to fore go an aircraft nut on it.