The coupler is used to mount fence posts to another post using clamps. The posts have an outer diameter of 35 mm which is kind of common in Germany. There might be a lot of stress on the part. In the use case pictured, it might very well be that the short piece of pipe has to bear the full weight of a kid while it climbs over there. So print it with a lot of perimeters. Whatever you do, print it laying flat with supports. The first one was printed with my usual "solid" profile standing up- it didn't survive the mounting process. I even used a modifier in the slicer so I get a solid piece of plastic internally that extends all the way from the tab with the bolt hole to the other end (see the pictures from the slicer). The couplers are attached to the pipe with self tapping screws.

Print instructions

6 perimeters, 0,3 mm, print laying flat!, use a modifier for contours to get a solid piece of plastic internally. Use supports.

I used PLA as a test to see how well it will hold up in the outdoors. If you don't like experimenting better use PETG.