I just bought a bag from Jan and planned on getting the clam shell going into it but talked to Jan about it and it seemed that the clam-shell was a lot of extra material especially if you weren't running the bag at full capacity.
She suggested the spider harness in lieu of the clam shell lid. I went that route and have no regrets. The nice thing about the spider harness is that you can use it to keep the bag in the stern bay. Sure the everything bag is connected to your D's and frame but in the event of a flip gravity would pull the bag and its contents down and out of the stern bay. The spider harness helps with this issue by putting downward pressure (when boat is right side up) by connecting to D's and then running through bag D's and being tightened down. On low-key runs you can leave it at home and just run with the bag.
I don't have any experience with the Pacific River Bag but personally I didn't like the idea of a zipper for closure. I would think the zipper could fail over time?
Check out a picture of the octo-harness being used with Jan's bag on her site. The spider harness is basically the same thing except with 6 strap "legs" instead of 8.
Cargo Storage, Cargo Thing, Everything Bag, Cargo Thing Bag
Note: I don't work for Jan but find her gear to be totally bomber. She is super easy to work with, very responsive, and ships wicked fast!