It's not a matter of another script, Image>Canvas size is in standard Gimp.
In practice I "invented"(*) the technique to help someone on a Forum That Shall Not Be Named. Much faster to explain (and perform) than telling him to download a script, install it and run it. It's matter of "bang for the click". Of course, if you need to to this every day, the script is useful, but people also request scripts because they think there is no other way...
(*) Going back in the thread history, I find that this thread started a long while ago, and this is exactly the same as Jolie's suggestion (first reply to O's question). Great minds,... etc