I'd like to know the answer to this question too!
I have a seatbelt harness but in the boot I havent a seatbelt clip and Dexter is too big for a crate so he is atm just in the big boot with a dog guard but I'm worried he has too much space in an accident and need something to restrain him!
Also have the problem that Riley out grew his crate. So I fitted a dog guard (not one with the legs that take up space in the boot) and bought a metal /chain (he chewed through a non-metal one
) lead. Put the loop of the lead round the base of the the middle headrest (it comes out making fitting easy) and then used the lead to secure/clip him when he is in the boot. Works a treat! I would be reluctant to secure him to the actual guard as it is possible a strong dog (and Riley is) could pull it away from its mountings. Also in a car accident (god forbid), the guard may dislodge and if the boot pops open you have a loose dog on your hands!