Do not make your own flails, you'll likely wreck something and/or get hurt and it won't work. The forces involved in swinging those around are tremendous, and when they hit something....

I've literally shaved granite rocks flat with a flail mower and didn't even nick a cutting edge, they're designed for "when things happen," and things do happen. BUT, you need to start with good equipment if you want good results.
The balance and weight distribution is critical, engineers have Finite Element Analysis software, years of experience, and they do lots of testing; if there was a better way to do it they'd be doing it.
Your better bet on making an awesomely clean cut is:
1.) Buy an awesome flail mower and at least one extra set of flails and an extra set of bolts&nuts (I have 2 extra flail sets, so three sets total: 1 to run, 1 to swap out when the first set gets dull, 1 to pull replacement flails from for when a flail gets damaged).
2.) Learn how to sharpen the flails.
3.) Buy the sharpening and measuring and weighing equipment and learn how to use it.
4.) Keep your flails sharp and mower clean and serviced.