Free 

I don't like the mounting setup for the Lego 31210 set - it requires two screws, and one of them would be partially visible.
Instead I made the following hanger piece that fits tightly into the Lego pieces and mounts with a single screw. If you print it in black PLA it will blend with the Lego piece it mates with (Technic 67139 Pin Connector Block).
Onshape model: https://cad.onshape.com/documents/8b987c305f443303cdc3a067/w/6f772c891c178a698bb5ba1c/e/636d39c250bed7649eca2f92?renderMode=0&uiState=6665b809b722c3448a908e50












Names are used only to indicate the source of a result. Polyzon is not affiliated with the listed sites.