Bought this in ebay fits snug.
https://www.ebay.com/itm/Dust-Moisture-Cap-for-GM-3-0-Duramax-Diesel-Silverado-Sierra-3-liter-3-0L-/304085771723?mkcid=16&mkevt=1&_trksid=p2349624.m46890.l49286&mkrid=711-127632-2357-0
Extremely happy. Bought 2020 white silverado 1500 diesel in September 2020. Have not had any issues so far 9100 miles. Lifted, blacked out, tinted windows. Fingers crossed.