I have started to design an expansion module, and I have been doing this using flux.ai. I thought I would would post a link to the template I created along the way, it is designed to fit into the 3d printed module housing that framework has released. I have not yet tested to ensure the fit, etc. so be warned.
https://www.flux.ai/tyler-s/framework-expansion-module-template?editor=pcb_2d