The second post in the thread...

... start off by filling the (foam) voids with bondo, sand and shape back to smooth and then begin the process of laying glass over the foam.

From what is seen in your pictures, you appear to be very close to a final size you are wanting to end with. This will be a problem as you lay glass and resin you will build up a considerable thickness. You should consider giving yourself at least 1/8 to 3/16 of an inch of room for glass build up and then final sanding and finishing.

I assume he's not familiar with the process AT ALL.
You're going to need bondo filler (as suggested) but also fiberglass cloth and a lot of epoxy. It's like paper mache, if you've ever done that.

The few projects I've done, I found it easier to build a "buck", which is dimensionally what I want ther finished product to be. And smooth, and etc. Perfect.
Then I pull a mold from that. Then I lay my 'glass in the mold.

Im not good enough to lay glass over a toi- small buck and then expect it to take shape for a finished product. (And it sounds heavy!?)

Sent from my SM-G970U using Tapatalk

Well, this is the correct way to do it. It will definitely give better results, and be repeatable if you make your mold correctly.
The way I believe OP did it is the lazy one-off way to do it.
I may be completely wrong in my assumptions though. For perspective, I've only personally used glass for repairs and reinforcements. I haven't made anything from scratch.

I'm not sure I'd call this the lazy way. This is how you'd build a one off item. If you want to replicate the part multiple times you do need to 'pull a plug' and then use the plug with the PVA/Release agent to allow you to lay up the parts and have them not stick to your plug/mold.

Nice project. I tip my hat.

First, thank you to all of the responders. Your comments help.

I can get to the stage where I have shaped the foam and attached it to the old vent using hot glue or two sided tape no problem. Bondo and shape to smooth inside and out is also a non-issue. This is where I struggle a little.

I understand Fiddlebog's reply that glassing is a bit like paper mache and I really have no issue with that and I assume it needs to be done certainly on the outside but also inside as well, correct? It's just going to be a little harder to finish it on the inside of the part.

Would love to do as Maytag suggests and build a part mold and I may try that but I only need one so using the original vent and simply glassing the foam scoop onto it is probably the best (easiest) way to go.

I will attempt to simply glass the foam scoop onto the vent and run the glass down 1/2 way on the neck or as little as possible so that the fit is still acceptable in the fenderwell. I'll show you the part as I progress.

Anything else I'm missing?