As I was making the cover, I noticed that when cutting bird's mouth joints on the diagonal boards that support the edges of the cover, the cuts weren't square in the front-to-back vertical axis. It was off by about an eight of an inch over 5 inches. Pretty bad. The table supports were a few thousandths different from one other and the support arm was too. And they actually lined up just wrong to multiply the error. Anyway, I put a couple broken off hacksaw blades under the support arm to act as shims, and brought the table into squareness.
Here, I'm pointing at the ends of the hacksaw shims barely visible in the picture between the frame and the table support. Using a hacksaw blade as a pointer.
And checking the squareness of the table to the blade now. It's still not absolutely perfect, but it's close enough. Maybe off by 1/16th of an inch over a foot.