Plastic Inspection Hatch Cover complete with sealing ring and retaining strap.
Available in 3 sizes: 4", 6" and 8": 
4" (100mm ID / 155mm OD)
6" (152mm ID / 205mm OD)
8" (200mm ID / 260mm OD)
 
Used on boats, caravans, fishing (live bait buckets) and storage containers as well as waste bins.