couple of tips to cutting or working with fiberglass (even insulation)
Talcom poweder use it rub it all over any exposed skin. it plugs the poors to prevent the little fibers gettin ito your skin.
Breathing protection WEAR IT!!!! I have done enough damage to my lungs over the years and this stuff is bad for you, its like asbetos in your lungs the fibers burro in to the lung walls and never come out.
I have made many tools for working with glass and some are pretty trick. if you have to make up areas where you have huge gaps use masking to tape to fab up the space and the glass it in, the tape come right off after it sets.