I'm sure my plastic dealer still has their scrap bin, but when I went to pick up some supplies about a year ago access to the interior of their store was restricted per local public health guidelines. Sometimes they'd charge me for scraps in the past, sometimes not, but never more than a couple bucks. As for the modelling compound to the best of my knowledge it is basically a paper pulp and plaster mixture. It dries enough to retain its form fairly quickly but I usually leave a piece out in the sun for several hours to help the process along. Depending on how wet you prefer to mix the compound I'd say a 1cm layer would give you about 20-30 minutes of working time before it firmed up and after a couple hours it might be dry enough to work on top of without major issue.
Right now I'm messing around with making a couple of wire tree armatures to go on these pieces, however they're not really turning out the way I want them to. I probably won't end up using them for this project. Thanks again for the kind words Clach Umha. I hope you enjoy the final product.