FWIW if I want to secure the saddle I put a D-lock through the frame and rear wheel paired with a cable to loop through the front wheel and D-lock. The cable is usually coiled fairly tightly around whatever I've secured the bike to, and then a padlock locks the saddle rail to the cable.

If someone wants to cut the cable they'll be able to steal the saddle and the front wheel but they won't get the saddle without either cutting the cable, or the lock, or the saddle rail. My saddle isn't worth enough for a thief to go to that kind of trouble.