They make an Omnibounce for the 420.
You could also use a plastic container and get similar results. You could also use a bounce card which would give you similar results.
The omnibounce is made to aim some light forward, while bouncing most of it off of the ceiling (or wall etc.) If you are not bouncing, it doesn't actually make the light softer because it doesn't really increase the size of the light source. It does make your flash work harder which reduces your range and uses up the batteries faster. The same is true of most accessories.
