Aluminum soffits and air traffic control

[MoparNorm]

Yes, the outside is finished; enclosed with aluminum soffitt. Anything I do must be done from the inside.

The Strongtie literature shows a frieze vent. This must be a comparable application to the piece that Sickcall posted. The Strongtie literature illustration leads me to believe that the screen was put in from the outside.

An interesting, related matter is that the building supply houses sell pieces that go between rafters and extend down toward the soffit, to keep that space open, free of insulation, so that circulation can occur.

Gordon,
It doesn't matter how you attach the vents, only that you have at least two of them and that the screen is small enough to keep out the flies. The other openings should be closed with blocks. It's not a good idea to use insulation, it will stop all air movement, which you don't want and just encourages other pests to nest. If you can't find the screens, just get some woven wire fabric (screen) and cover the openings.
I assume that you will insulate the ceiling that you install at a later date?
It's not exactly clear to me what you have here, was the "basement" constructed with a gable roof under the house? Why the rake or slope to the ceiling joist? It looks like roof construction rather than a ceiling. I'm guessing that the air space is uninsulated and open to the first floor above? If so, you need to come up with a way to allow the ventilation while providing an insulation barrier between the living space and the exterior. As it stands now it seems to me that (need more information) the underside of the first floor as well as this "basement space" is uninsulated and costing you big time $ in lost heating efficientcy.
MN
MN