The handful of times I've encountered a need to remove spanner nuts I resorted to tedious tapping away with a small flat bladed screwdriver and hammer. Its hard to get motivated to purchase a commercially produced version of a tool that used infrequently, so I decided to see if I could 3D print a reasonably functional one.
I had success printing mine with PETG-CF. ABS would be a good choice as well, but if you only have PLA/+ available, you can still get a working tool as long as you break the nut loose first with the good old screwdriver and hammer trick.