I wouldn't recommend this if children are likely to visit the area, but I once used wide roof flashing for a barrier.

I just made a trench with an edger & pressed the bottom edge of the flashing into the trench.

It has very sharp edges, so wear heavy gloves & be careful.

I did the same thing 2 years ago along one of my stockade fences..works great, no intruders and no escapees. I used a heavy duty stapler and stapled the top at 1/2 foot intervals to the fence to make sure no injuries would occur. It's held up very well. I have 3 kids, the youngest being 3 and they never get near it being that it is behind the plants against the fence. but even if for some reason they did, like a ball going back there, i am confident they won't get hurt with it being stapled to the fence. I highly recomend it.