I got this idea from a post in the DrZzs facebook group and finally finished the design. As of right now its -20 and there is 60cm of snow by my fence so I won't have any finished shots anytime soon. Once I do I will update this print.

I am using Prusament PC blend. I think eventually I will use ASA but but for now this is the strongest filament I have so I thought I would give it a shot. The lights are WS2811 bullet pixels. The cap is a 6x6 post cap from home depot. As long as the holes for screwing into the posts are the same any cap this size should work fine.

 https://www.homedepot.ca/product/veranda-6-inch-x-6-inch-rigid-plastic-post-cap-in-black-for-deck-or-fence-post/1000681910

The post piece slides into the centre of the inside with the blocks facing up. I am using weld-on 16 to glue the two pieces together.  The lights can be mounted, the cables run through the slots and then slid onto the post and screwed in. The top cap be put on and screwed into this adapter.