curtain-rail
Roll-up window shades come in certain widths x heights, 180×210 cm, 120×160 cm, etc.
They are made of some kind of fabric rolled up in an aluminum tube.
To adjust their width to fit your window frame you probable have to saw the aluminum tube and cut the fabric.
The biggest challenge is to make a straight cut, without releasing the fabric from the tube. It's even more difficult when it's a double layered plastic drape
Are there any tips in how to make the perfect cut for this kind of shade?
You're not showing all the pieces. What we see is a curtain rod and the center support.
There are brackets for the end that the rod hooks over.
Pictured is a dual curtain rod hanger bracket. The hole in the rod catches on the tang and it drops down onto the bracket.
The answer to your problem might be shown in the picture, use a nice board that spans across the window opening about six inches above. Use screws to hold it to the studs and then mount the curtain to that using stronger screws than came in the hardware pack. Graininess of the cellphone picture doesn't do the spar varnish finish on the hemlock board any justice.
For spanning that window (10') there are heavy-duty three piece curtain rods constructed exactly like these but with two of the screw-in supports available in addition to the end brackets.
Best thought I've got is to put a valance across the top of that opening, and put a middle support from the valance to the rod. Or switch to a 1" rod.
Best Answer
I would use my miter saw with a fine-tooth blade. Your question says "the perfect cut", so buy "the perfect tool"; which is a specialized window shade cutter: