This is not rocket science. Whatever the OLND you want, the total axle length has to be 9-10mm longer. Example: for 120mm OLD rear hub, you need a 130mm rear axle, that way you end up with 4-5mm of exposed thread on each end. I can post a picture if needed.

You can get one of these from wheels manufacturing, they are very tough and have never seen one break. You can cut them to whatever length you need.

Wheels Manufacturing Hub Axles