^^^That's a backpack. OP was asking about mess bags.

Ortlieb does make mess bags:
http://www.ortliebusa.com/prod-78.htm
http://www.ortliebusa.com/prod-60.htm

I've looked at the Sling-It's;was not impressed by the strap system,and it only has velcro to close it,so you can't over stuff. No interest in the Zip;the zipper closure limits how full you can stuff the bag,and they don't run very big anyway.