Burley Tail Wagon water bottle cage mount.

You'll need
2x M5 Nuts
2x M5x10 bolts (the kind you'd use for a bottle cage)
2x 4.8mm wide zip ties

Print with organic supports.

Press the two nuts into the nut cages put your water bottle cage on with the bolts, and it will just clip into place on the trailer like in the photo.

I've managed to keep them on for decently long trips with out the zip ties, but every now and then a bump, or the dog, would knock them off so I added cut outs for two zip ties to hold it in place.

Should work with any normal water bottle cage, ones in the photos were from thingiverse:
https://www.thingiverse.com/thing:4287976