if you don't want to modify your calipers, how about filing down 0.25mm off of each side of nuts?
you obviously want to make sure you get the angle correct for the slot and wheel.
that's pretty much how some older hubs are, with two flattened sides on the axle.
That would possibly work with the Shimano or Tektro holders but not with the Kool Stop ones due to different design of the mounting hardware. I did widen the slots on a pair of Campy Monoplanar Chorus brake calipers but it just seemed like it would make collectors cringe and I wondered if there was an "off the shelf" solution so to speak.
None of the road caliper brake cartridge holders use a full diameter bolt fastened to the holder. On the Kool Stop ones the nut has a 6.25mm diameter extension that fits through he caliper slot. On the Shimano and Tektro cartridge pad holders there is a 6.25mm diameter floating nut fastened to the cartridge holder with the fastening bolt being 5mm diameter so there is not a lot of metal in the sidewalls of the nut.