Using thin aluminum and either a blade or tin snips, you can go 1 of 2 ways. You can either wrap down the sides or make it solely to cover the top. It's more difficult for round fence posts. For rectangular you can cut a line from the edge of the metal to the top edge on each corner and bend down. For round, cut multiple lines from the edge of the metal to the beginning of the post and bend down.
You can also purchase rubber fence post caps. I'm not sure how costly they are but I've read several studies that show they are much more effective.
Looks like you can purchase them fairly cheaply for example
Or you might get some ideas for doing it yourself from this post https://www.madisonartcenter.org/#diy-copper-post-caps-by-your-own
You've gotten some good advice. I think it might be easier to make a cap out of wood and perhaps give it a waterproof coating. Here's a good tutorial for you: https://www.youtube.com/watch?v=ejGA7cI9bI8
If you're set on aluminum, you might want to go with Scarlet's suggestion of buying the caps.
You could always use Tin or Jar Lids maybe? Best wishes
The cost and time for material cheaper just to buy one.
They aren't expensive. Have you tried the home improvement stores (Lowe's, Home Depot) and amazon?