It's a pretty simple design, created in Blender. The only challenges are managing the wire-feed path and making the dimensions somewhat precise, given the small size.
Tight corners make it harder to pull the wire through. 24AWG bell wire works well. The wire also needs to sit a little proud of the surfaces to ensure a contact with various shapes of battery-holder.
Prints fast and easy - try it out. The STL file is shared on this project page.
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Created a Tindie product to sell these to interested parties. Just went with a dollar plus shipping for minimal bother.