How not to drive with blocked rear-view mirror?

I’m planning a trip this June, and I will be driving full capacity (5 people) for about a six hour trip. I really doubt with all the luggage that will be involved, that I will be able to see the cars in the rear-view mirror. I don’t really want to buy a roof rack, or a camera. If I have to, I guess I have to, but anybody know of an alternative? Of course I won’t know if this is really an issue until I pack, but honestly, I see no way this can be achieved. I do not, under any circumstance, want to drive for 6 hours with no visual as to what is behind me. I’m driving a Jeep Compass, if that helps.