I use a chop saw. Wrap the cut in a tight wrap of masking tape to support the fibers, then a couple of close, adjacent wraps to help ensure squareness against both planes of the fence. Wear safety glasses!, a respirator, and hearing protection. If you can set up a vacuum at the cut end of the dust stream, it is worth the effort. A HEPA vacuum even better. Hold it tight against the fence. Take up any slack on a slide saw. Cut once through and let the blade come to a complete stop before letting up, or the blade can catch the cut piece, and ruin it at the same time as tossing it really, really fast. Let the blade cut, not slow, but not pushing it. Go take a shower and wash your face carefully. The saw should have a decent, high tooth count blade, and be aware that fiberglass will dull the blade quickly. I would not use a nice blade that I care about, but only nice blades that I do not care about.
However you cut it, very fine particles of the size that are most damaging to your lungs do not really settle out of the air, they remain in suspension for days. Control the dust as it happens. HEPA is your friend. Protect your eyes.. both from the fine, invisible dust, and the accidental things that fly.
