It's going to be hard to find a flat sub grill. The majority of them are raised so that there is clearance for the subwoofer's excursion. If the grill is too flat or shallow, then you wouldn't have enough clearance. So every time the bass hit, your sub would extend out and hit the grill. Probably the most shallow grill you will find is going to be the waffle style grills which probably came w/ the sub.
As far as Heavy Duty goes, Dayton makes a very sturdy and affordable grill. You will just need to make sure that the sub has enough clearance during excursion. Here's the link:
http://www.partsexpress.com/pe/showd...number=262-862
Another option is to use a grill that will cover more than just the sub. You can find a size that covers the subwoofer and also extends out onto the sub box itself. Then you would simply mount it to the sub baffle. Since the grill will be wide, this will help eleminate that "round bump", you are seeing right over your subwoofer.
http://www.partsexpress.com/pe/showd...number=262-870