The top of one of our 4" wood fence posts was rotted out, so I made this customizable cap to place over the top and improve both looks & structural integrity without replacing the post. Default dimensions in the attached files are for a 4" square wood post (actual 3.5"), but you can customize to your liking. Rectangular posts are supported -- does not need to be square.

Optionally, you can use customizer to change the top bevel angle, remove the back, and/or notch out the sides (to slip over existing 2x4's for example), and/or add supports to the sides. See some examples in the attached STL files.

The SCAD file includes an optional round finial, but you can disable it by setting "sphere_finial_radius" to 0.