edited

If what you need is a flange for infusion then a flange is what you need to make.

If it were my problem I would take a sheet of aluminium and make a funnel shape that slides over the outside of your top lip of the mold on the left picture. Put a few bondo dabs on the inside to secure it to the bold temporarily

I would then fiberglass THE OUT SIDE of the aluminium. Once cured demold and what you have is a funnel shape in fiberglass that is smooth on the inside. (Of course wax and use parting agent PVA first)

Now bondo it into position from the outside . Carefully rough up and fiberglass the funnel to the mold lip from the inside.

You will have to fair it when cured. Use glass matte cut up in about 1/2 inch or smaller squares and use them as filler to bring up low spots.

Now that your surface is close apply gelcoat over the faired glassed surface (inside). Make numerous coats until you are higher than your intended finished surface .

Wet sand to blend the two parts of the mold and polish.

Bing! If you use the same color gelcoat it will look like it was made that way in the first place.

I have done this a number of times and it works well. All depends how much you want the mold because it is a bit of work.

Fred

Cheers, have got a good idea now.