The part you're looking for is typically referred to as a "Dust Cup Filter Cover," "Dirt Cup Flap," or "Dust Bin Flap." It's the component that prevents dirt and debris from escaping the dust cup during vacuum operation.
Finding the specific part number for the dust cup flap of your model is proving to be a challenge. The search results mainly direct to general replacement parts pages or vacuum sales listings, without specifying the part number for this particular component.
I'd recommend giving Dirt Devil a call. They may be able to tell you what the part number is for a replacement.