Squaring Extrusions? =(

I'm in the initial build stages of the C-beam XL, and am squaring the extrusions. To make one, just one, totally square took at least a couple hours. I have the extrusion clamped to my bench, and I'm using a file to remove metal. I'm checking for square with a Moore & Wright engineer's square. Should it be taking this long? Also, how square does it need to be? Am I looking for perfection here?

I did see a clever idea on squaring with a table saw from @Metalguru and may go...

Squaring Extrusions? =(