I think most covers will work well, I have an old Sitka cover that works well on my 4800. I also use it to sit on when the grounds wet. It doesn't fit super well if the pack it packet full with the collar and lid full but still keeps it dry, with the pack in daymode or full + lid without the collar filled up it works great.
Kifaru sells a pack cover too, personally I would just buy whatever is the cheapest, I've only used mine a few times in the last 10 years.