My Holan racks have taken 5 or 6 falls in gravel without any dents or major nicks on the bike. Of course, this isn't going to help you since Holan no longer has an NA distributor
I'll second the recommendation to go the "trash can" route for luggage. I picked up a pair of sea horses for less than 100 shipped (don't remember the exact price). They've held up well over the year and I think I'll continue to keep them. They are fairly light weight, really waterproof and can take a heavy beating.
Rethink the weight part for ammo cans - they can easily easily top 10-20 kg per box, vs 1-2 for the plastic options. That can translate into a miserable ride, especially since the weight is out on the sides.