Please make sure to understand that while using onshape links to export STLs you are effectively pulling unstable and untested parts. Those are links that guide to live documents that change as EVA is developed.
Each project page contains a Onshape link to it's Onshape document. From there the parts can be exported to many different formats.
Please remember that it's done under the GPL-3.0 License.
For now if you want to contribute reach out to me on Discord. Let me know what's the idea you and if it'll fit what EVA is supposed to be you'll receive a repository in the EVA-3D Github Organization, from there you will be able to maintain your mod accordingly.
The EVA 2 / Spec page may come in handy.
Your contribution is always welcome, even if it doesn't conform 100% to the eva spec, for that there's the EVA / Contrib Page where you'll find a guide to submit your contribution. Thank you!