720's rock! I bought an overkill rubber mounting kit and ended up just using 4 bolts, lock nuts, and 1/4" flat washers. It is on a small SW Motec AL rack, so it's foot print is in the middle, about half way across. I drilled 4 ea - 1/4" holes in the AL rack so it would be a tight fit, marked the holes in the bottom of the 720, and bolted on. It holds about 11 lbs max (anything on rear rack is that weight capacity), but I load it with a few containers of liquid groceries sometimes for short distances, when my duffle bag is full.
The main things I store are my helmet sack, bike cover, knee pads, a couple of emergency tie-downs, green tire-flat goo can, and little odd stuff. For $85, it came with steel locks which I use outside my garage. And laying flat on the rear rack, it cuts through the wind nicely.
And being an efficacious industrial engineer, I pointed it opening up towards my seat so I save about 10 steps every time I use it: as well as don't step out in traffic in parking lots. Seahorse 720 w/metal locks - $85, hardware - $2, rear al rack - $180, and 40" long x 40" dia large duffle bag - $70 = $337 complete modular rack system for groceries, laundry, etc... Well, $20 of visibility stickers on the box too, and I go to Planet Fitness for $10/mo to help me carry up to 150 lb. loaded sacks on my back