OK, I managed to install and tame the new magnetic support and I'd like to share a few thoughts.
Firstly, installation of the magnets wasn't at all easy. I guess the problems I had are rather individual phenomena than universal ones. My hand is too big to put it through the sound whole and I had to ask for help. Additionally, since my guitar's sides are narrower at the upper part (the one closer to the fingerboard) there is less space for the hand to get in deep enough in order to position the magnets. But I managed.
Secondly, the support is really adjustable virtually to any position you can think of (well, OK, any I can think of). Thanks to that, whether the magnets are 2-3 cm more to the left or right, it doesn't matter that much. You will be able to set the thing the way most comfortable to you. This is very helpful.
Thirdly: there is one aspect that could - in my humble opinion - be improved. Namely, the width of the leg support. I understand the logic (and logistics) behind the width used but if I had a choice between the more portable version with the current leg support and a more, say, "deluxe" one with a wider support, I'd definitely go for the latter.
Lastly - I'm not sure if anyone mentioned it so far - the Kris Barnett magnetic guitar support is esthetically pleasing. It looks great, it feels nice, so when you take it to your hands you know right away you're holding a high-end, top-quality product. When attached to my guitar, it looks as if it were originally made especially for my instrument. The two fit together so well.
Aha, one more, important thing: the magnets do hold well despite the thickness of my guitar's sides. Owners of The Queen of Guitars by Rafal Turkowiak may safely use the support. It works with no problem.
Mr. Barnett, thank you. Your inventive mind, skillful hands and top-notch work make my moments with the guitar even more enjoyable. Life can be so good!