FacelessTech designed this small board to act as a surface-mount edge connector:
SMD edge connectors
So you want to joint two boards together, You decide to use through hole female and male 2.54 pitch headers. Normally you would use through hole than ether have a 90 degree headers or just bend them over. For years this is how I did it, I would have to make sure I made sure the holes were just the right distance from the edge of the board.
The KiCad design files are available on GitHub:
OSH Park 