Run a bolt [round headed bolt] and nut [ nut will act as the nub] thru the fin and get spring straps......
I would reinforce the rubber material around where you make a hole for the bolt maybe use some wide washers too. If the nub came off I'm doubting the quality of the material or forming on that fin. I never heard of that happening before. But if they don't replace the fin it is fun to see if you can jerrybuild a fix.