This is a pair of jigs for centering a post-cap on a 4x4 fencepost. My caps were 5.5x5.5", from Home Depot.
The jigs will help you to drill centered, horizontal holes with a regular power drill. The models are sized for 3/8 bits and 10mm dowels. The smaller jig is sized for a 4x4 fencepost, and the larger for the 5.5" post cap.
To avoid support in the print, I designed the drill-through cylinders as separate inserts. Print two of them, and glue them in (I used Gorilla Glue). If you have smaller/larger dowels, you only need to update the insert.
Once the holes are drilled, you can just hammer the dowel into the post, and then hammer on the cap. I used wood glue on both sides (although in retrospect, polyurethane caulking might have been a better choice). Since there is only one dowel you can twist the cap to align it with the fence, before the glue sets.
I guess you could also replace the insert with something more creative and make your own fully-printed post caps.
Designed in Autodesk Fusion 360 (f3d archive included)
Printed on a Replicator 2 and Low quality setting. I got a bit of curling on the larger guide, which just barely fits on the build plate, but it didn't really affect the use of the jigs.
For smaller build plates I included a cropped version of the larger guide, which only has one corner. This is sufficient as long as the caps are square.