When my Dad bought the 69 Road Runner that I now own, someone had installed a bicycle water bottle on the radiator support of the car and to this day, that is what I use. I really like it. At any time, I can take the tube out of the top of the bottle, slide it off the support and pour coolant out if needed.

Whoever did it simply cut off the nipple that you drink from and installed the overflow tube in the hole. It fits tightly; you'd almost think it was made especially for this reason.