Old houses were built this way. Soffits were only used on more expensive houses, for the architectural look....Use what I would call a vent board. cut a 2x4 the length of the distance, between the rafters, or joists...If your carpentry is precise, they should all be the same. I measure each gap, regardless.
Then, using a hole saw or a paddle bit, about 1 1/2" in dia. bore 3 holes in each board. Cover with your desired screen, preferrably on the inside.
Nail board into place. Hopefully, before the roof is in place.
You may have to rip these boards down, slightly, if you have cut a bird's mouth into the rafter, which is proper. I rip them down, with the angle of the roof, so that I have full bearing, across the board. Hope you understand.