There are several ways to unwrap an object, if you choose to mark seams, you need to create as many seams as necessary, and put them where you want Blender to cut off the UVs, as if it was peeling an orange. For example here I've cut along all these edges, then unwrap, and it worked fine:

[![enter image description here][1]][1] 


  [1]: https://i.sstatic.net/9uaW1.jpg