which bike? Do you have any other luggage space? I have a small bag that has the power pack, tyre pump and a few other bits and bobs. Takes up a small portion of the larger pannier on my V85TT Travel.
If you have limited capacity, maybe a small waterproof bag from someone like Lomo that clips to the handle bars, crash bars or on top of the topbox? Assuming you have an overnight bag in the topbox, sling this bag in the topbox at your hotel parking.