Here are two accessories you might want to buy for your car's roof basket.
1. Clip-on lights
You should consider purchasing some clip-on, battery-operated lights for your roof basket. The reason for this is that when you take road journeys late at night, having some illumination in this area of your vehicle can be very helpful, as even when your car's headlights are on, they won't provide any illumination of the vehicle's roof.
Having this nearby light source will mean that you'll be able to easily unbuckle or unclip the tie-down straps that were keeping your possessions in the basket in place and can use both your hands to do this quickly, instead of having to hold a torch or your smartphone in one hand to light up this area. Similarly, if you need to load the roof basket at night, these clip-on lights will allow you to see well enough to distribute the items neatly and evenly across the basket and tie them down securely, even if it's pitch black outside.
These lights could also prove useful in the evenings when you're camping or at the beach and want a light source, but don't want to keep your vehicle's headlights switched on in case they drain the battery.
2. A roof basket cover
The other accessory you should get is a roof basket cover. Many car owners don't buy this item when they first purchase their roof baskets, because they only plan to use the basket for things that don't require much protection (like their bicycles, hard-shell suitcases, skis, etc.). However, most people with roof baskets will end up getting this accessory at some point, as it can be very useful for transporting items that require protection from the rain and wind.
For example, if you're going camping and are taking several sleeping bags and mats that won't fit inside the car and need to go in the roof basket, having this cover over the basket will mean that these items won't be soaking wet when the time comes for you to sleep on or in them. Likewise, if you're moving house and need to transport your clothing collection as well as lots of other items, you can strap your clothes onto the roof basket and then use the cover to protect them from the elements whilst you're driving. You can also use it to protect any furniture you need to transport when moving.