Hi Steve – I see you’re looking for more flexibility than you can achieve by using existing keys as a base.
I don’t know Blender, but I ended up using Solidworks. Its an advanced program, and I had to get several lessons to learn how to use it and model in 3D. Are you starting from scratch? If your software is capable, it might be a case of more learning. Keys aren’t that complex, so it should definitely be possible.
I used digital callipers for measuring. I made the keys to fit my own melodica design, so it was quite easy to get an exact match. But even when they matched in theory, there were many obstacles with regard to air tightness, accuracy, getting a smooth key depress, evenness of key layout, weight distribution and key rigidity (which effects responsiveness).
You could try scanning the keys with a 3D scanner, and then tidying them up?