I can only offer suggestions from my personal experience, but maybe that is good because niether you nor I will likely learn to fibreglass like a jouneyman.
The two ways I tried was
A) tape a flat peice of something tightly against the outsid surface of the hole you want to cover. It should be firm because it is what will hold your new fibreglassing in line with the outside surface, and if it bends out when you put the glass in then you will have a lot of resurfaceing and bondo work. Tried that.... no fun
if the taped surface stays flush with the outside I would start laying fiberglass from the inside in reverse order. A layer of light
bondo, then pour a little epoxy down to hold the next layer of glass against the bondo, then if you need stuctireal strength I would slapp on some of that magic Hair Bondo. As far as timing goes I would let each layer start to set before putting the next layer down. Tried that more fun....
C) cut a piece of fibreglass canabalized from some where else, tape it in place and fibreglass the seam , the a layer of finreglass mat. After it sets you can bondo the other side where it needs it. If things go right you moxt of the finishing work shouod be along the seams tried that too
I have attached some photos of what I did with my firnace holes left when I removed the furnace
, on the outside wall of the trailer I employed option B, and on the inside back dinette seats I changed the hole to meet my requirements for a long opening. I chose option A for that one.
Fibreglassing gets much better when you stop letting it push you around. Like everything often the only thing resting between success and failure is confidence.