+1for the bootbags. I have one and it holds plenty, does not obstruct the high brake light. I have put press stud poppers on the flap that covers the zip though. Just a belt and braces job to make sure the cover is there when the bag is full. I also use a couple of cheap ratchet straps as extra anchoring but not a necessity. Extra stuff is behind the seats or in the passenger footwell.
I like the idea of two bootbags for added stowage.
We also strap collapsable seats and a small collapsable rable on as well. Makes weekends at TOTH etc a comfy day.