Is this the whjite circle for a Swastika? You could try a 1/4 " paper punch, although it may be about 1mm smaller than you need, but close. Either buy a sheet of white decal film ( and you'll have enough for a lifetime of applications), or scrounge an unused decal with enough white space on it from your decal stash. I did this for an air recognition Swastika on a Z class destroyer with small Swastikas from an aircraft decal sheet and red decal sheet cut to size. As an alternative, you could cut a mask and airbrush the circle.
