I have a question that I hope to get a quick answer to so I can continue building tonight 
I've seen that you should (?) use wind deflectors in a cardboard-like material that is available for purchase at most places. It sits between the rafters along the wall so that loose fill insulation doesn't fly straight out when spraying. It also probably has some ventilating function.
Can you use, for example, building film instead? They weren’t super cheap per piece either, so I was wondering if it’s okay to use something else, I will have enough building film left over.
I've seen that you should (?) use wind deflectors in a cardboard-like material that is available for purchase at most places. It sits between the rafters along the wall so that loose fill insulation doesn't fly straight out when spraying. It also probably has some ventilating function.
Can you use, for example, building film instead? They weren’t super cheap per piece either, so I was wondering if it’s okay to use something else, I will have enough building film left over.
What do you mean by so-called wind diverters, is it a board that sits on the rafters and an air gap of about 2.5-3 cm against the roof sheathing, and seals at the eaves?
Google wind deflector, unfortunately can't upload images via phone
OK I have not heard the name before, just luftspalt, it is a stop in the eaves so the insulation does not fall out as well as luftspalt against the roof decking, you can take whatever you want and do the same thing. One usually nails a strip of about 25 - 35 mm on the rafter against the ceiling, then set up a board of any material on it, (if the board is a bit flimsy, a strip may be needed fastened to the board so it does not bulge when they blow in the insulation), against the ceiling as long as there is a gap. Then there's the stop in the eaves, it is usually placed between the cap board on the wall and against the board in the ceiling, so it becomes tight. I have seen others tape the connection so no gaps are formed, this works in precisely the same way as the expensive paper things, then it's up to you. Hope this is helpful information.
Wind deflectors are the ready-made solutions with corrugated cardboard that create an air gap. Isover has a wind deflector called "lufta," paroc has another solution. Forms a 25mm air gap. It's easier to set up if the rafters have c1000 - 1200. Saves time but is more expensive than battens + hardboard.