Any powerful drafting software is designed for professionals and has a significant learning curve. If it is robust and feature rich, it probably costs money as well. Generally speaking, you get what you pay for both in time and money.

Drafting by hand is the simplest thing that might work; is compatible with Linux, Mac, and Windows and you can start work today. Good luck.

As someone who uses such software for a living, i concur. Hand drawings work just fine. Not to mention, any fabricator worth their salt will have no problem interpreting it.