For this I use tonneau cover for my pickup. I have a Toyota Tacoma and since I often drive off-road, I try to make the trip comfortable and safe not only for me, but also for my luggage. I advise you to read this article https://truckpowerup.com/how-to-keep...-in-truck-bed/ , there is a description of different types of tonneau covers. I recommend you to pay attention to hard tonneau covers. This will not only protect your luggage from rain, but also be able to protect your luggage from robbers. This is simply indispensable if you travel long distances and can stop on the road in a roadside cafe and not worry that something will pick up or ruin your things.