When I HAVE to use my on-camera flash, because of lack of good lighting and a slow shutter speed cannot be used because of a human/animal subject that cannot stay still long enough - I simply use a napkin, sheet of folded white paper, etc - anything white, to hold in front of my flash. You can try several shots, if still too bright, fold the paper or use more. This really helps reduce the glare and harshness of the onboard flash. It may be a little underexposed, but that is much easier to fix than an overexposed photo.
For those times that I need a bright light, because the flash still is not strong enough, I use a very bright shop light or even one of those long florescent lights, either can be found at any hardware store or walmart.
Hope this helps some...