Apple no longer sells either the iPod Universal Dock or the adapters used to fit the various iPod models into the dock. Consequently the adapters are very difficult to obtain. A link to 3D models of several adapters was recently posted on Reddit. However the posted Nano model would only work for the first generation iPod Nano. I found an online service to convert the posted STP file into STL in order to import the model into TinkerCad. I then elongated the opening to accommodate my 5th generation iPod Nano. This is a very amateur mod, but it works.
The author remixed this model.
The original model was for the first generation iPod Nano. The model was modified by elongating the cavity to allow the iPod 5th generation to fit into the iPod Universal Dock.