This topic has come up a few times, see these posts
//heartlandowners.org/showthread.php/29929-mpg-covers?highlight=cover
//heartlandowners.org/showthread.php/28655-MPG-custom-cover?highlight=cover
//heartlandowners.org/showthread.php/26023-Trailer-Cover?highlight=cover
Custom made covers are not cheap, we were quoted $680 plus shipping (100+). Ended up buying cheap one on ebay which seems to do the job fine. Link is here
http://www.ebay.ca/itm/Deluxe-4-Lay..._Camper_Parts_Accessories&hash=item4ab9ec0fe0
Shipping was $54 and it fits ok, bit loose at the ends but added some tie downs and has survived strong winds ok.