Yep, that was exactly the same problem I had: too much heat remained in the piece which allows it to warp. Which gives Marvin a 'lumpy' ass.
Printing two, like you did, already helped me a lot too, but I don't always want to print two, so I got that duct (get the one without holes, or you'll have to tape it), get a fan that moves a lot of air, solder it to the Flashforge or to a 12V adapter and you're good to go!