What kind of plastic container can I use to hold a detergent soak solution?
You can use any container that is robust enough to hold typical aqueous solutions of chemicals. A polypropylene or other chemical resistant plastic container such as HDPE (just about anything but polycarbonate or polyurethane) will work just fine for cleaning. If you have specific residue concerns, you should consider the possibility of certain metals or monomers leaching out of the plastic. For general purpose laboratory and medical cleaning this is typically not a concern because the anti-redeposition properties of the detergent are sufficient to stop anything that leaches out of the container from depositing on the substrate being cleaned.