Just an update to this thread that may help someone. I broke the Fantastic fan vent cover on my 2010 Hawk. My fan is a 4000 series. Unfortunately I didn't find this thread earlier. I did some research and found out that the replacement part number for that fan is K1020-19. I ordered it from Amazon, and found (like above) that it is dome shaped, not flat. I tried to install it anyways, but it would not close around the spacer wart properly. I called Fantastic Fan customer service, and they told me it should fit since "all the domes fit all the fans". They said there is a two year warranty on parts, no mention of a lifetime warranty on domes.
I found out that the correct part number for the flat lid is: 4020. The flat vent cover kit with the installed hinge, etc, in smoke would be: K4020-19. The Atwood part number is 90643. The retail price is around $29, a couple of on line RV dealers sell them.
An update to my update: I called Fantastic Fan again and spoke with a very helpful lady named "Bre". She said the dome shaped vent cover could be used to replace the flat cover, but that the lift arm would have to be changed since they are different for the flat or domed shaped cover.
Regarding the warranty, the domed shaped cover has a lifetime warranty, the flat cover only has a seven year warranty. Fortunately, my fan was dated "05/10" so I was still within the warranty period. I had to pay $15 shipping for the new fan.