I used 1/8" plexiglass. The fan is 5 inches square. I determined the length needed for the fan to project outside by an inch or so and the other end to extend over the popper. In this case the length was 14 inches. So I needed a piece of plexiglass 14 X 20. Making sure everything is square, I used a grease pencil to divide the plex into quarters. On the last quarter, I removed a piece off the end equivalent to 5 x 6. Using a heated metal strip (especially for bending plastic and at 300 deg F.) I bent each quarter into a channel or tube. Using the correct solvent for the plexiglass (available from the same supplier of the plex) injected into the seam with a diabetic syringe (needle). If the edges are clean and true the solvent will flow only between the held-together sections and fuse the seam.
I cut a 5X5 piece of plex and fused it to the popper end using the needle and solvent. Mounting the fan at the other end posed a problem and will depend on the fan used. I cut triangles of plastic, drilled holes for the screw mounting of the fan. Screwed them in place so that each corner had a 1/8" overhang. Pushed the motor into the end of the tube and sealed the corners with solvent. Next problem is getting to fit your window. I used an "L" shaped piece fused to the bottom of the channel to raise it up enough to clear the popper by 4 inches. I used two "L" shaped pieces fused back to back to from a projection that would fit into the side slot in the window. Closing the window up against the tube and covering the remaining open area with another piece of plexiglass.
If you can't get a bending heat strip, you could cut four pieces and butt edge fuse them to make the channel (good luck!). Cutting the pieces is real easy, just like cutting glass. Use a sharp razor knife and score the plastic on one side a few times, then take the line to the edge of the table and snap off.
Another idea is to watch for old display cases thrown out by stores. Quite often they contain a variety of plexiglass pieces and with any luck you could find all the supplies for nothing.
Use a belt sander to clean up all the edges that are sticking out beyond
the opposite edges.
You will need to find a 110 volt fan - it doesn't have to be very strong
as the popper will help things along as well.