Waterproof bags have existed for centuries - waxed fabrics, well-treated leathers, and I'm sure plenty of more modern textiles. Or just put a plastic/metal box

Waterproof bags have existed for centuries - waxed fabrics, well-treated leathers, and I'm sure plenty of more modern textiles. Or just put a plastic/metal box on the side/back of your bike and use it exactly like you do the trunk in your plastic/metal box of a car.