I drive a similar size SUV and routinely leave an empty cooler in the back for taking home groceries. I use it and sometimes cardboard boxes to wedge things in so they can't move. Even things without wheels can shift if you don't take some precautions. We were told in a driver's safety class one time that people have been killed by things as small and light as a tissue box. If it's flying at 60 mph when you suddenly stop, the force is more like a thrown brick. You don't want to even think about a loaded sewing machine bag flying through your car.

That said, we have a cover for the cargo area, which was optional equipment. I'm ashamed to admit it's been in a closet almost from the first month after we bought the vehicle because it reduces the vertical space back there and makes it harder to load things, but it's still a good idea because not only does it help keep things in place, but it would make it harder for a potential thief to see anything enticing. I don't know whether this is something you could get for the Outback, but it might be worth asking the dealer. An earlier SUV we drove when our kids were young came with a stretchy package net that hooked onto the sides. So that might be another option that the dealer could supply, or they might even be available as generic items in auto supply stores.

I seatbelt my machine into the back seat when I travel.

I'm a bungee cord user also, easy to hook and unhook. Before I seat belted my machine case.