Please don't cut corners on the quality of your tap and always use the proper sized drill for the tap.
This is an "F" drill for a 5/16" X 18 thread tap. You can also use a 17/64" drill with a 5/16" tap. Fiberglass or epoxy sheet is susceptible to off center tapping so be careful to not cut crooked or wobble your tap side to side as it will make the tapping bigger than you want. Practice in a scrap piece of fiberglass first if your not comfortable with this.
Once tapped through use an air gun to blow the flutes of the tap out. Do this before reversing it out of the fiberglass..