Probably going to have to make it yourself.
There are places in town for the plexiglass or acrylic.
http://plasticworksinc.com/
You need to make it yourself. I have been down this road a few times now and ended up just cutting a hole in the bottom of the casement window frame (it was much larger than the one in the picture as it was more like a door in my case). In the winter I put the plug back in, covered it with gorilla tape, and put a kicker plate over it so it looked nice without compromising the insulation.
The cheapest/easiest is with plywood but obviously that's ugly. You may not care if it's just a temporary solution. Lucite / Plexiglass is what you want and you can buy sheets at Home Depot or Lowes. Cutting a hole in the middle is a bit tricky, you will probably have to start by drilling into it (VERY carefully, if the bit sticks, it will crack the whole sheet), and then a jigsaw after that (again high speed, but moving very slowly).
There's no way I'm making that myself. I wouldn't even know where to start.
Guess I'll see if I know anyone that can do that or even plywood since it's only for a couple of months.
If you don't care that much about the aesthetics (or more likely your wife/GF haha), just use plywood. Way easier and way cheaper for the 2 months.This quote is hidden because you are ignoring this member. Show Quote
I’ve had 2-3 of these made over the years. Find a plastic/acrylic shop and give them the measurements of the window along with the size of the duct.
How about something like this?
https://www.amazon.ca/dp/B07CCMZ3FD/..._H.x8CbRN7CB1F
Interesting product - also has half decent reviews too.This quote is hidden because you are ignoring this member. Show Quote
Thanks, might give this a try.This quote is hidden because you are ignoring this member. Show Quote
Ive just used plywood in the past. Painted white to not look as ugly.