With the hard bumps the rear end takes, no tape or glue is going to do it (for long). But before it falls off entirely, it's going to dangle on the wires for a while and customize the fender and paint, badly.
It must be bolted down.
Looking at the picture, it may be possible to do so with narrower bolts and washers, going through the holes crooked. Or drill.
Or, go back to how it was, and get some flush mount led signals for the rear of the bags. And that requires an electronic flasher unit or they will flash very fast.
