I had to do that on a Fireball Classic I rebuilt and sold several years ago. I did the same thing Sunking suggested. Just filed off enough of the flare do get the standoff through the hole and then I cut a clear plastic washer to go over top (under screw head).
Those are a pain.
As an after thought, I know a couple of machinests who told me they probably could have done it like the original if I had brought it to them. Do you know a machinest?
G