I did that once. Create some white shape with soft edge effect. If that's not enough, blur the whole layer. Set to about 75% opacity, layer blendig mode to screen. If the blur still isn't soft enough, enlarge the layer. Remember to check the render.
Yes, use a mask.
AS 9.5 MacPro Quadcore 3GHz 16GB OS 10.6.8 Quicktime 7.6.6
AS 11 MacPro 12core 3GHz 32GB OS 10.11 Quicktime 10.7.3
Moho 13.5 iMac Quadcore 2,9GHz 16GB OS 10.15
Moho 14.1 Mac Mini Plus OS 13.5