Hi e77g,
welcome to the forum! Regarding a Subaru XV roof storage solution, there are a few simple and inexpensive options that can connect to the existing rails without investing too much. For example, soft roof bags that are easily tied and can be removed immediately, or compact roof boxes with simple installation.
If the use is low and the budget is limited, I recommend going for a soft roof bag or lightweight roof storage — this will both save you money and allow you to easily remove when you change cars.
Good luck!