I'm not sure everybody understood your question due to lack of exact vernacular
For describing these parts. Just explain your question in more detail unless you did in fact get your answer. Bike make. Photos of parts would help too.
I think you are talking about your dust cap that goes under the coaster brake arm. If so you need to grind down the edges of the cap to fit so it does not on contact your kit sprocket. Then the arm needs to be bent in sort of an s shape so it clears the sprocket as it turns. You should use the altered dust cap to keep crud out and grease in. you do need the rubber between the kit sprocket and the spokes as Venice said and as crassius said you could make a cap out of plastic but they don't last very long based on personal experience as he said. This linked video should answer all for kit sprocket installation.
If your talking about a Cnc sprocket wrong answer...sorry/ ask again