@mikelga Thank you, happy holidays and new years to you too!

Metal wouldnt be any colder than plastic, it would just change temperature more quickly. But after a night of cold their temperatures would be the same. Also to get really big crystals youd want it to not be too cold, just barely freezing for an extended period of time. For this reason the plastic likely helped this effect as it insulated slightly from the cold for a little bit allowing the water to cool down more slowly.