Pretty sure you can achieve that by using the WINDOWS MAGNIFIER, in windows 10 all you have to do is open the windows magnifier, go into options, switch the magnifier mode to fixed instead of fullscreen, resize and position the zoom window, check "follow cursor" then find the middle of your screen, put the follow cursor check there, uncheck follow cursor, and you have a zoom OF the middle of your screen IN a little window wherever you put it.
I suggest setting the Zoom Level BEFORE freezing it in the middle, as when you change it, so does the point it zooms towards.
Freezing the place the magnifier is linked to is also necessary, as games don't place your cursor in the middle all the time.
Hope this helps you out. I was looking for the same thing you were, but couldn't find it anywhere, tested this in a few games and since it's "always on top" it works quite well.