My mind is slow this morning, but the principle of option 3 above (flare nut/brake line spanner) combined with the shape of a standard cranked ring spanner would accommodate cutters of any diameter. The gap between the jaws would slightly need to exceed the shank diameter of the cutter.
What would need to be measured/verified is the distance between the underside of the cutter and the top of the nut so the spanner can go in sideways and then drop over the nut.
A standard cranked ring spanner could have a section of it cut out to this dimension and would still grip on five flats of the nut.
If the crank angle of a spanner is to shallow, cut the wall out of a 1/2" drive socket (a 1/2" square drive will fit over a 1/2" router shank) and weld a piece of flat bar to the socket. That would give you a low-height solution.
If you cut two entire flats out of a six point socket, effectively you are making it open-ended so the tool could be even shallower, never having to go downwards, only sideways.
