Giant 20' x 3' x 2' router build

[EddyCurrent]

. . . it would be nice not to have to do anything to it though. Many thanks

I don't have a water cooled spindle but if I did this is something I would try.

Cold water header tank fed from mains water via ball cock, feeding small central heating pump (you can regulate the flow on these), return back to header tank.
Also an electric cutoff valve could be included, operated from the cnc control panel.
If the temperature got high I would include in the circuit a small central heating radiator, handy in winter.