You can buy cables with loops on both ends. Pass one loop through saddle rails, then through other loop. That gets you that end. Then put your u-bolt or other lock through the loose end when locking it up. Store cable with lock, rather than on bike, to avoid flapping/rattling. It won't secure the seat POST, but it will save your saddle. Like the previous commenter, fairly thin cables are OK, since you aren't going to defeat someone with cutters.