First thing I did to my Si sedan was install a dog hammock in the back seat. I have already been on a 1,000+ mile disc ("Frisbee") golf road trip with my 55-pound mixed breed travel buddy. The hammock catches dog hair and keeps the dog(s) in the back seat, while still giving the dog(s) freedom to sniff the breeze (horse and dairy farms are especially interesting) or curl up and sleep (usual highway activity).
One side of the hammock can be rolled up for a third passenger, or the whole thing can be removed easily if needed. So far it has held up well.
The hammock does not restrain the dog(s), so if restraint is the goal the hammock will not suffice.
I use this car kennel I got off Amazon. There's a strap in the back of the kennel that you can stick the seatbelt through to buckle it up. I'm using the kennel strap that straps over the head rest instead of the seatbelt/ seatbelt strap in this photo because I have the seat protector on. I don't let my dog roam freely due to safety issues, and she also sheds a lot and rubs her nose all over my two windows in the back, lol. Got lazy of cleaning it everyday.
I measured it out and it is long enough to work from the front seats and out the back of the open hatch to cover and protect the rear bumper. I'll take pics and let everyone know how it works once I get it.
So this worked out really well; our first hiking trip went great!
This "hammock" thing is a really great idea, I should have looked into these earlier... Before she would always try to stick her wet nose between the front seats and get to the front/walk over the console/shifter area, and now she seems pretty happy/snug back there.