Um, wait a minute. I may be from Canada, but at last count, there are 35 million other folks up here as well. But none that I personally know who make window louvres (and notice we spell differently up here. It's a British thing, what with centre, favour, Liberals, and all that rot).
But there ARE two "Keiths" on this site, and I believe it was Keith Lundholm (of Tennessee, aka opelspyder) who made and sold those rear and side window louvres a while ago.
Actually, I DID make a rear window louvre for my GT about 25 years ago. I had a used louvre from a '74 (?) Celica Fastback, made with aluminum slats, and I just cut off the ends and re-formed them to fit the GT's rear window. It wasn't very hard, and I suspect that the basic materials are readily available. Six pieces of aluminum (4 inches by 38 inches), a central spine (24 inches by two inches), two peices of aluminum to hold the edges together, two stainless steel piano hinges, and a couple of small locking pins (to allow the louvre to be lifted off the rear window for cleaning) and you are set. The angle of the Celica's rear window was about the same as the GT (now this was a "Fast back", versus the coupe), so the centre spine didn't require altering. Yours can be adjusted to suit your particular GT's angle. I used a block of wood, with a curved edge, to bend the ends over. Or you could just make a sharp bend. Aluminum pop rivets hold it all together.
Here's a couple of photos: