I want to put windmill on my fence cap, and I found many nice windmill on makerworld:

• Toroidal Windmill
• Honeycomb Windmill
• Elliptical Windmill
• Sunflower Windmill
• Raccoon Windmill
• CLAMP for Windmills
• https://makerworld.com/en/models/425419?from=search#profileId-329027

So I added a base for these windmill. Also added screw holes on 4 sides to secure it.

The windwill rod is easy to push in and out, you can use some superglue if you want to make it permanent.

The fence cap size fit for standard 87.6mm around 3.44 inch. If it doesn't fit, I also provided a print profile for you to do your own scaling.