nealtw
Contractor retired
If he has access to the framing below the sub floor will be in the way, but he could cut a hole so he could use nuts and bolts for the straps.O.K., so now you know I have too much time on my hands. I'm on the injury list so I have to stay busy somehow.
Another solution might be to use metal plate, 3/16" would be more than plenty. You could probably get by with just the one plate on the front of the post as your railing would hold it from moving parallel to the stairs. And, a wide plate on the front would hold it from moving side to side because the plate would be in shear. Hear again though, it still depends on how well it can be anchored to something solid under the carpet. The plate wouldn't have to be thick, as I mentioned you would be putting a shear load on it, and it could be hidden by routing a cavity in the back side of the wood you box it in with. If you can pull the carpet up you could chisel a 3/16" pocket in the floor to eliminate a carpet bump. A matting plate under the floor and it would be solid.
If you're lucky enough to have a floor joist in that location to tie it off to that would be great.
There is smaller system made for this. Kit 505
https://stairfasteners.com/s_newel.htm