About the only techniques that will tolerate scrap plastic involve sand casting. Thermoset plastics can be shredded and suspended in a low temperature low viscosity to reinforce it. Thermoplastics, assuming that the type of plastic is matched, can simply be melted and poured. Keep in mind that the resulting product will be much weaker then commercially available plastics.

IMHO, the best skills for self reliance involving scrap plastic would be bonding (glues and fasteners), heat forming (bending, not melting), and mechanical forming (files, drills, and saws).