I had one of my fence post caps go missing, and I wanted one that would match my others (at least in size and shape) so this is what I came up with. It almost exactly matches the cheap wooden fence post caps that I got at Home Depot. If it was printed in wood PLA it would be even closer.

It is designed to go on a 4x4 post, and it gets screwed in from the top with three 3 ½" deck screws.

I've added a square on the bottom to help center it on your post.

I printed mine at a 0.3 layer height using a 0.6 nozzle and 10% lightning infill. It still wasn't exactly a fast print. Print with NO supports.