300 - 400 yards shooting means you already have a scope mounted, and I see it's a day scope not a night vision one since you're asking about visible light. Among visible wavelengths Red light is invisible to animals, so it won't disrupt the foxes when pointed at them. You also need long throw to cover the range, Aspherical Lens flashlight is good option, Klarus FH10 (white, red, green) / about 45mm lens diameter is good, but its max throw on Red is only 225 meters, if longer throw than that is needed, you can check out Dereelight Xsearcher with Cree XP-E2 Red LED / aspheric lens diameter is about 66mm, it should do the trick. Another one is Wolf Eyes Seal with Red LED.
As far as Reflector construction flashlights, I'm not aware of any dedicated throwers with Red LED, maybe EagleTac has Red module for some of their big reflectors midels like M25C2 (not sure of the exact model name). Anyway, you can always use any reflector-based flashlight and put a Red filter on the front, like Acebeam T21 or Olight M3XS-UT (both models has Red filters accessory options), or any other similar rifle-mountable flashlights that has Red filters available (Fenix, Klarus ...etc), but that will be less efficient approach and consumes bettery power faster than using a flashlight with Red LED.