I have a 2005 MB and the skylight cracked around the edges when it was just over 1 year old. Called LD to buy a replacement cover and Vince told me that the skylight cover was warranted by the manufacturer for 2 years. I contacted at Ventline in Bristol, IN and they sent me a new cover.
The cover was Ventline VA0123-01. It is described as a "20 inch x 20 inch dome".