Here is the source for the 2U Connector part: https://cad.onshape.com/documents/eecb39d3cd81220d3a56ce06/v/fdc9aca356206986ef6440e6/e/f881e9547bb3cd4fd653d041

You can export it in other formats

You will also need two additional frame sides

Here's how to upgrade a 1U CubeSatSim https://github.com/alanbjohnston/CubeSatSim/wiki/Upgrading-to-2U