Under you seat. In your saddlebags. In a storage box. If you can fit is somewhere that you can reach easily, then that’s the place to put it. Luckily, modern motorcycle rain gear is lightweight, compact, and stores down into a small transport bag or something similar. There’s always room for it somewhere if you’re inventive enough. If in doubt, a good bungee cord can secure it down tightly on to the back of your seat or on a luggage rack. If in doubt, you can always carry it around in a special backpack if there’s rain forecast, and leave it at home if the weatherman says it’s going to be clear all day. It’s always good to have you rain gear on your bike, just in case you get caught out by an unexpected downpour.