Tape the sides of it with first some masking tape and then some thick tape. Then file that down with a file. Goes very quickly.
But it is fibreglass. So I would wear a dust mask, and have the vacuum cleaner with HEPA filter running next to it while I do it. Then suck up the dust. Also do it on top of a newspaper, which I then fold and throw it away. Clean desk/workbench with vacuum cleaner. Then with a rag. Then clean my hands. Change clothes. Then take the dust mask off.