I based the design on the connector for the NCL>NCA lower engine. I presented the piece to Steve at Origins. After looking at it, he pronounced it good - but requested that a metal pod be created and added to the adaptor, truly making it a one piece conversion.
I created a metal replica of the standard Federation cargo pod in brass and green stuff and attached it to the adaptor. This is what the final piece looks like:

More photos are available at the link in the signature below: