After I got tired of slow and tedious process of making accurate perforated angle with the drilling contraption, I figured I'd do it with an inexpensive drill press.
Having made more than 8 ft of perforated angle on the drill press, I still was not completely satisified with the accuracy. This time the errors were mostly in centering of the holes. I guess aluminum angle jig that I made bent differently depending on the applied load, and so the holes were drilled in slightly different places. Oh well. It looked like a more rigid jig would fix this problem, or..
Or I could just forget about the accuracy requirement, drill larger holes, and make adjustments during assembly. I don't know why this thought hasn't crossed my mind before, especially since I encountered a couple of situations where it would have been nice to have some freedom to move a component. I had even modified the motor mount to have 1/4" holes for the motor instead of 3/16".
I took some inaccurate perforated angle made previously, enlarged the holes to 1/4", and bolted together a perfect right angle corner. Duh.