ABF UV Unwrap
UV > UV Create > ABF UV Unwrap - An Angle Based Flattening UV unwrapper with the ability to split geometry along selected edges for more complex objects. To use it, select a sequence of edges on your object and hit the button. There are no options.
Known issue: Meshes with multiple open borders can cause the resulting UV Map to become unsymmetrical. To solve this temporarily cap holes prior to running ABF UV Unwrap.
Use Rectangularize Grid to square up your UV map