I had some hassle getting Sketchup to recognize the .rb scripts, but after five restarts it did.

So I made a 36“ by 12” square, used Offset to cut out the center with 2“ walls, and then used the Volume calculator.

I got an answer, 1.73 cu ft. But I also got a warning that I had ”Ambiguous Form“.

So apparently this calculator doesn't like hollow objects. Are there any other scripts that might tolerate hollow objects?

BTW, the official instructions on installing scripts are wrong for OS X: it says to use the plugins folder inside the Sketchup package when it should say to use the plugins folder in /Application Support/Google Sketchup6