I made my own from some plastic coated steel cable from B&Q, it comes in various sizes, I use the 1/8th cable.
Cut the legth you need plus another foot.
Get some metal tubing about two inches long, that three peices of the cable with go through tightly.
Thread one length through then bend it back on itself forming a loop big enough for a padlock to go through, pass it back through the tube then bend the end back on itself again into the tube and hammer the tube flat.
Do that on both ends and you have a length of plastic coated cable with a loop on each end which can then be passed through the handle of the gun case, or action of the rifle and padlocked to either luggage retainers in the boot or any other secure part of the vehicle.
I use this method to secure my gun hard case to radiators when staying in hotels etc.