First off, Congrats!!!! Secondly, I wanted to answer your question. I used the widest file in the set, laid the file on a table and then rubbed the end of the clip across it length-wise until most of the arch was taken away. I didn't file it enough to get the square edges, but you could. It would take a while for sure. There might be an attachment on the Dremel that would work, I just didn't try it. For what it's worth, I did not have to re-drill the holes.
Just reach out if you have any other questions.