I've discussed this with a neuroscience PhD student, and his answer was that you'd need to buy the helmet, or make one with plans from someone who really knows their stuff. He mentioned OpenBCI. (We discussed this a few years ago, but the small electronics scene hasn't changed much.) It looks like that kit costs $600-$850. There are other options, if you search for "open eeg".
The problem is in the analysis, according to my friend. How do you target what types of brain signals are what you want? My friend indicated that setting it up for biofeedback was pretty much infeasible without the help of an expert. I suspect you can find more detail on this problem in biohacking forums.
As for the instructable you referred to, I couldn't find any comments that said they made it and it worked. However, I did find comments pointing out serious flaws in the explanation of how the device is supposed to work.
Edit: Muse makes a headband and meditation app which does some amount of biofeedback, but having used a few times, I still can't say whether it works or is just detecting calmness (which is absolutely not the same as focus, nor is it the same as what your biofeedback is looking for).