I would look for a screw that fits the threads myself.
That said, you can make a sort of "sideways U" out of a piece of sheet metal (by bending it), and tap it for the threads you have. It goes both above and below the lower bobbin flatwork, so the screw passes through it twice. It leaves the original bobbin unmodded, as it is reversible.
A method easier than that, but which modifies the bobbin permanently, is to just epoxy a small square of tapped sheet metal or drilled-out fiberboard to the bottom of the bobbin, below the original screw hole.