Shaker hole caps can be found in craft stores and are good for capping hole drilled for adding screws, There is a stud right behind the post in the wall you can screw into and a couple screws into the floor framing below is good.
https://www.woodparts.ca/mushroom-button-wood-plugs.html

https://www.woodparts.ca/mushroom-button-wood-plugs.html
