Really the best way to do it is similar to a floor in a house... cut the hole bigger until it spans to the middle of a stringer on each side to give it something to sit on... Then personally, I would grind a bevel on the existing deck and the new piece of wood to create a "valley" at the seams to allow room for buildup of new glass, this way you won't end up with a big obvious hump at the seams where the patch was made.
There might be a better way, but thats what I would do.