Honestly it is probably best just to replace the case back. You may be able to source one from a TD and they come up in the M2M parts from time to time. The gens are set with a gasket just like any other crystal. I would assume that the reps are too, but I have never had the need to pop one out so I am not positive. My concern would be why is it loose in the first place; damaged gasket, bad machining, misshaped crystal, etc. If the crystal was seated improperly or is crooked, you could remove the case back, place it on a flat surface and apply even, downward pressure to see if you can correct the issue; This is assuming that it is indeed set with a gasket. Beyond that, the cost to repair would probably be more than picking up an entire replacement case back and without a press you are going to have a hard time doing it yourself. To try to glue/silicone really isn't a viable option because no matter how precise you are at applying the adhesive, you are going to see it.
