Storage for bottle cage mounts?

I bought a new mountain bike recently, and it has three sets of bottle cage mounts. But there's no way I could actually get two bottles inside the front triangle, and the third set is on the bottom of the down tube where I don't think I trust a bottle to stay anyway. So I have one bottle on the top of the down tube, but the other two mounts are currently unused.

I was using a Dakine Gripper on my old bike, but it never stayed in place, so I was hoping to find something like that that would mount directly to the bottle cage mounts. But I can't seem to find anything like this.

Does anything like this exist? What do you do with extra bottle cage mounts?