I had planned to use a band sander. Put it on there and let the thing 'wear' it down. Thought that would be easiest, to get straight edges, evenly long on all 4 sides.
I took a metal ruler, and scored a line on all 4 sides using an exacto knife, to know where to shorten it to.
Then i found that the sharp knife did a good job, and ran it along the groove it created some more, until it was all the way through. Took a while, but not that long that i regretted doing it this way.
The rounded corners were a bit fiddly to do, but since i was almost done, i just kept on using the knife.
I did that first on an original Hasselblad shade, which is made of pretty hard plastic.
I did it again, the same way, on those Chinese hoods, and it took no time at all. Pressed too hard on one, and the side cracked. So take care.