Watch this tutorial to have an excellent understanding on how to control the Shape Tool to create corners that you really want.
Be patient and watch closely.
1:10 Tutorial Objectives
1:50 View/Default
2:00 Import image into Inkscape
3:07 Object / Align & Distribute
3:32 Path/Union
3:40 Shape Tool In Action
6:02 Gradient Effect [G]
7:20 Saving File
7:28 Exporting File (+ Export Settings)
