I like the 1-qt. rectangular metal cans that paint thinner, alcohol, etc. comes in. Solder a strip of galvanized metal to the can, paint it black, and a couple of sheet metal screws can mount it just about anywhere there's a flat spot. Keep the can as close to the level of the radiator cap as possible. The rubber tube from the overflow needs to reach the bottom of the can, or close to it.
Jerry

Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!