Keep in mind that simply by putting something opaque in front of the flash, you are not necessarily making the light any softer. The softness of light is determined by size and distance to the subject. So the point of things like 'the puffer' is to increase the size of the light source.
Either way, when you use something like this, you block a fair amount of light and also destroy the beam like properties that it has coming from the flash head...so your range (power) is severely reduced...and the pop up isn't very powerful to start with.
My suggestion, if you must you your pop up flash, is to just use it as is...and maybe play with the FEC if you think it's too bright for your situation.