Stangaholics has them for $439.  Can't speak to the quality.

https://www.stang-aholics.com/i-24197238-65-66-mustang-fastback-fiberglass-shelby-style-spoiler-style-deck-lid.html

a friend of mine , who is building a 66 FB, bought a pair of the fender extensions with the spoiler parts made into them, if memory serves me right they were fiberglass. he had a spoiler made out of metal and welded on the trunk lid. it turned out really well.
 

I think making your own would probably cost as much or more than buying one.  The learning curve can be steep with fiberglass as well.  The shops that are making them are using permanent molds, chopped strand guns, etc.  You could certainly build a mold out of wood and make a one off, but my gut tells me you would have several hundred dollars in wood for the mold, resin, etc.  That assumes you have all the woodworking tools needed to build the mold too.  There are also special tools needed to work with fiberglass on this scale (metal roller, etc.). 

With anything fiberglass I would be prepared for fitment work.  Its very rare a fiberglass part just bolts right on without issue.  If you're dead set on this course I would go with the one from Stangaholics as recommended, and factor in the cost of paint and fitup at the body shop of your choice. 

Jim Bowie wrote:

I had gotten mine from Meyer Racing  I think back in 2008, It was for 66 coupe, had the end caps blended into rear quarters, paint guys did spend alot of time block sanding lid, still looks as good today, 

Even the best fiberglass is going to need a lot of blocking to be smooth due to the way it comes out of the mold (the body shop will factor that in, they know glass needs a lot of blocking).  Meyer has a good reputation as well, so that's another source you can try.

I have been looking for a nice original 66 fastback decklid for over 25 years now.   If you do go fiberglas, and have an extra 66 decklid, and want to sell, let me know!

There are two fastbacks currently at a restoration shop near me right now. Both have those decklids.  Just my opinion, but neither one looks right.  I have seen others that looked pretty good, on a couple of race cars I have seen.

I like the idea of making your own metal decklid spoiler.

I like all kinds of metal fabb'in (it's why I am a big fan of Daze)....The VMF thread metal one...looked like the fiberglass one!
I like!!

One question.........where does the rain water/car wash water go? Only question I have ever had about these types of spoilers.
6sal6(jus say'in)

Awesome thanks for the link. That is kinda what I’m talking about doing. I don’t have a ton of fab experience but I can weld and I’ve got some determination i think I’ll see what I can do before deciding to drop that amount of cash. Ill add it to my project list. And will post pics to keep y’all updated.

Thanks