I've added those with custom plastic recipes to my todo list, those being Castra, Igrys, Paracelsin and Cubium.
Moshine and Tenebris make plastic bars using the vanilla recipe so you can use the regular molten plastic recipe there.
Frozeta is one of my favourite planets but sadly I'm unsure how to implement molten plastic there or on other scrap planets, I'll ask AnotherZach about it but can't promise anything.
Cerys is not only scrap based but also more of a puzzle than a planet. Having known Thesixthroc for almost a year, I've seen him create and balance a carefully curated playthrough that I'd hate to disrupt with a custom recipe. Seeing as you don't have access to molten metals while being cargo locked there I think it makes sense for you to not have access to molten plastic either.