I prefer to do this with vectors. A rounded rectangular can easily be created with the shape tool (Rounded Rectange Tool), which also allows you te select a radius for the corner in the option bar. Make sure the paths icon is selected in that same option bar when you draw the shape.
When you're done, select in the menu
Layer / Add vector Mask / Current mask and it will create a mask based on the current path. The path can be easily adjusted to get 2 rounded corners instead of 4, just make sure that you make use of rulers, guides and snap which makes the whole process much easier.
Just use the method you prefer, I'm just offering an alternative.