A plastic article useful as a riser assembly for a septic tank or as part of another structure is comprised of a multiplicity of identical shape conical wall plastic risers. The risers are attached to each other at joints by tabs which engage rim segments; each end of the riser has alternating tabs and rim segments. A riser assembly wall undulates in the lengthwise direction. The risers nest within each other for shipment or storage.

An insulating disc can be easily installed and removed from inside a riser of a septic tank. The insulating disc is seated atop an opening of the tank lid and seals around the riser inner surface. The disc can include a core formed of R-10 extruded polystyrene sandwiched between two layers of high-density polyethylene. A flexible gasket can extend outward from the disc to seal against the inner surface of the riser. Retention tabs can also be provided to center and hold the disc in place. The disc can be divided into two semicircular halves that are hinged together. A hook, eyebolt and/or rope can be used to aid in removing the disc from the riser.