I need to use a 300mm mini-access chamber inside my extension (where 2 soil pipes meet) as screwfix supply (floplast brand), which is the usual type with plastic risers and plastic lid. It is not spec'd as double-seal. The plans required double-seal. Would the usual approach be to cast a separate double-seal manhole cover into the conservatory floor ? This would give the extra peace-of-mind of actually having three seals ! Simon.