1/4 of substrate. I would not add a carpet.

Just make sure the actual tank bottom is within range so if the snake makes contact with it no issues will pop up. If you get the tank floor to 90 and then add a1/4 of substrate the hotspot should still be at a good temp and not present any danger to the snakes.

You do have the tstat probe outside the tank? I sandwich it between the uth and the cage bottom.