I gotta say that I have never needed anything other than a fingertip to hold the chainring nut. Once the allen loosens on removal you can just spin the screw out while putting pressure on the nut with your finger. On installation the reverse works fine. I have never had a problem with the nut spinning when I torque the bolt head once the nut snugs in.

I suppose my next rebuild could prove that wrong, but I've never seen a reason to buy a tool that does something I never needed.