I have an aluminum time trial frame (Teschner) that I've owned for several years (bought used) and the bottom bolt for the seat tube water bottle mount has always spun freely. Never really used any brain cells on it as I've never needed to ride this bike for longer than one water bottle would last.

I have now however been bamboozled into riding the bike leg of a 1/2 Ironman relay, so I'll need that second bottle. It turns out the boss spins freely and I managed to remove the bolt by clamping down on the edges of the boss, but odds are if I put a cage on, it won't tighten down adequately and I won't be able to use the same method to remove it.

So, question, is this a rivnut? If so, is there a way to secure it, or otherwise remove/replace it without huge expense?