More flexible than silicone sealants, these are designed to expand and contract when sealing dissimilar materials as well as keep out moisture and dirt. All can be painted after reaching full strength.
221—Use on ceramic, metal, and plastics. White and black colors meet NSF/ANSI Standard 61 for drinking water systems.

Available at McMaster Carr.

The PVC sheets are also available there in thicknesses up to 1 inch.