Avenir Seat Locker 3mm feet
This is a 3mm cable. Steel cables of all kinds are inherently insecure. They prevent casual theft and they can be cut with wire cutters or or the smallest bolt cutters or even scissors. Small saws cut through them easily as well. Any lock attached is likely able to be defeated with a hammer or picked or broken by anyone who knows how and has some practice.
What they do provide is protection against people from picking it up and walking away with it. Seats and other accessories are generally safely secured with cables because they are not the primary target of thieves. A thief who sees a good bike, and approached it, and finds it too securely locked for his particular tastes, may take things attached to the bike, such as lights, seats, bags, etc, especially those with quick releases.
My bike is a Folding Bike, as as such, it is pretty easy to take apart. The saddle is not special, however, it has a seat post rear rack attached to it, which has a Rack Light Rear Mounted Light attached to that, as well as a Folding Basket. All together, that is a lot of useful stuff which I bought and I do not want to see it walking away.
It is easy to use and there will be a variety of ways to use this cable. I secured it to the frame by putting wrapping it around the frame and putting the large loop through the small loop, and pulling that tight, and then wrapping the cable around the underside of the saddle frame, and then securing the loop to the rear underside of the saddle with a Master Lock 121T, which is small, secure enough, covered in a black coating to match the bike's scheme, and comes as a keyed-alike set, so I can use the other lock for this bike as well, and use one key for both.
I am very happy with this small cable. I do not think it would be secure with a thicker cable, as anybody will to cut this likely will be able to cut a thicker cable. If one wants to secure one's saddle more absolutely, it would be better to secure the bike itself or to remove the saddle when the bike is unattended.
I have a picture of it being used upload to secure my saddle. This cable can be used to secure any accessory, such as a basket, helmet, bag, etc, provided one can attach it to the item.