Buy copper nickels by the ton made of 75% copper and 25% nickel. Each increment is 1.1 tons or $1,000 face value of nickels.
Nickels are the conservative buy to copper pennies as the weight of the coin is greater than a copper penny so there is more metal, but the face value means your cost per coin is greater (five times so).
Of course, there is no need to sort nickels by metal content [yet] so the premium is less percentage-wise. You also have a safer base floor value in your investment given the five-cent face value.