I don't know what's available in the USA but here in NZ I used some contrete reinforcing steel in a jig I made up when I built my Brodak Legacy. It needed a slight clean up with some sand paper but was night and straight and very cheap.
I did the same thing except I used large aluminum 90 degree angle available at Lowes, Home Depot etc.
I cut 2 pieces...cleaned up the edges then zapped them together back to back.
Then drilled the holes for the dowels at the same time to ensure they were in the exact same spot on the angles.
Next I marked the same end of the angles to identify the orientation.
Then I knocked them apart and zapped them into position over the plans on the building board.
My building board is a Blanchard ground piece of steel channel that is definitely flat.
Works great!!