[mickmate]

Take a die grinder or drum sander and put a bevel on the top and bottom of the hole edges. That way the repair has something to bond to and the setting fibreglass won't pull too badly across the surface. Tape across the underside as suggested and cut the fibreglass cloth or mat to just fit the hole. Do several layers to build up to the thickness you need. It's not hard just a little messy and time consuming.