Thanks to you both. We ended up spending more time than I’d like to admit in the screw aisle at Lowe’s conducting tests with both the screw and the metal bracket.
For the record, the correct answer is 10-32 machine screw. The included screw is 1" in length. We bought 1-1/2" and 2" versions and ended up using the 2" as it made it easier to grip and position the screw in the center of the hole in the base, though the 1-1/2" likely would have been sufficient.