This device should assemble fairly easily. Once prepared properly, that parts should fit together fairly easily. The ring is articulated to allow for easy use. Some parts may require some light sanding. The tolerances are pretty close to allow for parts to fit snugly.
After cleaning parts of any build artifacts, fit the two ring halves at the rounded end. They should fit cleanly and can be rotated partially. Verify fit and mobility before installing ring pin. Once the ring halves fit properly and move freely, slightly sand the top of the post, rounding it slightly to allow it enter to small hole barely. Once the tip can be inserted in the hole, line up the three parts so the pin is aligned with the hole. Use a small hammer or other object to strike the pin, forcing it into the hole. Use caution keeping everything aligned. I was able to do it with a single quick strike! The ring should be able to be opened partially.
The rectangular "lock" slides over the posts of the closed ring. Light sanding may be required to make it easy to install and remove with minimal effort. The goal was to make it fit very snugly and is to help keep things in place while installing the main cage.
Finally, the cage can be inserted on the top of the post, using the key-like piece to lock it in place. The "key" will require some sanding to make it easy to install. A standard, thin zip-tie can be used in place of the key. I just wanted to provide everything you would need for a usable device.
WARNING: Take care to remove any supporting framework your printer may have created to support the cage. Leaving any sharp pieces behind could lead to a very uncomfortable fit! Use at your own risk! Use common sense. Make sure everything fit together and can come apart easily BEFORE use.