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Canadian pennies prior to 1997 were approximately 98% copper and 1.75 % zinc. So, a pre-1997 Canadian penny (aside from being illegal to smash in Canada) should give you a decent result after elongating. Note: Do not attempt to elongate a "Loonie" (the non-round coin). It will squish far too short and you will loose part of the design.
Mini M Containers work great for storing your quarters and pennies when going on a squishin mission. The containers holds exactly 40 quarters and 20 pennies -- enough to roll 5 sets of 4. You can prestack them (2 quarters and penny, 2 quarters and a penny, etc.) then just slide the number of coins you want to use out in your hand and youre ready to go.
The length of the tail also depends upon the coin that you use. A pre-1982 copper penny will typically be shorter than a post-1982 zinc penny because zinc is softer than copper and will elongate more. However, you could run 50 copper pennies and each might have a slightly different tail length because the amount of copper in the pennies will also vary. Use copper for short tails and zinc for long tails.