Yep, torque meters are as simple as that. Several plans available, which is best depends on tools available to you and your skill set. I don't mind soldering, so I used a different design, but that will work.
I find 2 in-oz for the full 360 range of the dial to be good for SO. Smaller motors tend to break around 1-1.2 in-oz, larger around 1.8 in-oz. For a good plane, launch torque will probably be in the 0.4 to 0.8 in-oz range, depending on ceiling height.
A 0.020 diameter for the torque sensing element of the design (the free straight length between the anchor and the pointer). That calculator can be used to iterate the length to a desired range. I set the units at 2, diameter at 0.020, and varied the length till 2 in-oz was as close to 360 degrees as I could get it. Around 9.3 inches.
Note, there are a lot of assumptions under that tool, if you want to understand it better, go to the referenced site for more details. Example, it assumes a steel wire for the stiffness coefficient. It assumes that you pick a wire size and max torque that does not permanently deform the metal (exceed its elastic limit) at 360 degrees twist. (0.020 in diameter wire for a 2 in-oz full scale range is OK)
Be warned though, I've found that torque meter hooks made from wire as fine as 0.020 tend to bend/open with use, sometimes releasing a tightly wound motor at a very inconvenient time. Two possible solutions I can think of. One, double the hook part back on itself to the pointer and bind it togehter with fine copper at the pointer. The other is to solder a thicker piece (say 0.032) onto the sensing element at the end, making the hook from the thicker wire. That's what I did. Make sure the free length of the sensing element is the desired amount between the anchor and where you solder it as that 0.032 wire won't twist at all compared to the sensing element.
Comment on calibration. You can find procedures on the web, I think one is described on the Free Flight Fantasies site. But frankly I'm not sure I'd bother other than as a demonstration of procedure to the students. I've found the calculated scale accurate enough, and careful building has allowed my meters to be comparable to each other within usable accuracies. You don't need much beyond two to maybe three digits of precision for these things to be invaluable. Of course calibration DOES let you compare to others. Which is a key element in science.