Technically if the parcel shelf isn't in then anything in the boot could become a projectile in the event of a crash and injure the passengers. You can get dog guards that go behind the headrests that are custom fitted to the car so really really solid. We use these for our dogs. Look at travall or guardsman.
Also the fox 5 is great but it's not a compact fold at all 🙃 I have the venicci tinum and it's great off road and a one piece fold when using the seat.
It get a metal dog guard personally x
I have a bugaboo Fox aswell, and unfortunately my parcel shelf had to come out as I have a vw polo! It’s annoying but the only way it works unfortunately! I put the base in first then bassinet on the top, it stays there safely!x