If you cover any of it up you will lose the coverage pattern and could end up with a stripy shot. The best bet is to use, as SW says an ND Filter. Any make will do, and it doesn't need to be in great condition as there is no optical path involved. You can fit it to the flash with stick on Velcro strapping or even elastic bands. Give it a go, it works. One filter should cut up into two or even three strips. That gives you with a 2x ND filter, 2x, 4x and 8x factors.