Never thought of the knobs when I hard mounted my bags. Just keep a small crescent on the bike, and if they need to come off, I'm done in about 2 minutes.
Looking at the exploded parts diagram, it's a nut / bolt combo (8X16 bolt) for the four screws on the sides. Unless you just want to keep it all metric, you should be able to find some standard size stainless knobs/nuts that fit the hole and go with those.
Found this site w/ some stainless metric knobs-studs.
J.W. Winco Standard Parts Section 8 - Knobs