You don't mention what bike? Either way, most Bosch internal fitments consist of a fixed bracket at the motor end & an adjustable one at the top of the downtube. Almost certainly it will be that one causing the play. Take the battery out, loosen the bracket bolts & move the battery bracket downwards a little & retighten.
Blimey thats the first dual pack I have seen in the wild, prob cos they are so expensive. Must be crazy range, is the internal battery 625Wh?
I'm not sure whether that frame has the later type central rail for fixings or it has another method? Either way, there must be some bolts somewhere as has been said, I would just try and adjust them to hold the batt tighter. Maybe we can help further if you can take a pic of the mounts with the battery out.
With the MRS rail loosen the upper bracket and slide it a bit down (or maybe up) the rail and tighten it again, maybe you have to remove the rubber strip for a moment.