Whan adding bog (epoxy with microballoons or carbon) as filler, cover with fairly stiff but flexible plastic to get it level and smooth- taped down. Epoxy won't stick to polyethylene (visqueen): if you aren't sure, tape down a thin sheet of shopping bag polyethylene or painters' drop cloth first.

Q: Why sanding inside the edge of the hole? So that a flexible backing plate (greenhouse fiberglass sheet glazing is good) covered with wetted-out 6 oz cloth layers will stick snugly and well to the inside of the hole. Then it will be recessed just enough to fill from the outside with maybe 3 more layers and be very strong.