So you are going to create an interface but dont want those ugly corners?, well heres how to get rid of them easily.
1.Open a new file any size you want, use the rectangular selction tool and draw out the size you want for your iface. Keep the selection alive.
2. Create a new channel and fill it with white.
3. Deselect (Ctrl+D) and add a Guassian blur, set it to 10.