After three tries, I now have a functioning (though not yet manufacturable) prototype of the 'Tech-Sight aiming device':


Shown on a dismounted rear sight used for testing, but the wife ran ten cadenced shots through a 10/22 with it mounted last night and it stayed firmly in place, so that's progress. As you can see from the first image, the rear sight is clearly visible, and I had no trouble picking up the front sight. I could easily see breathing pattern, muzzle flip after firing, and return to the NPOA position. Would work really well with dry firing or a ball&dummy drill. Less directly useful for determining proper sight alignment, since you can't actually see the shooter's eye position. OTOH, the device creates a 'tunnel vision' for the shooter that makes it very hard to not notice that aperture (sometimes a problem with kids).
On the down side, the attachments consist of two super magnets and a piece of spring steel that are just hanging out in the wind right now, not a good idea for something to keep in the range bag. And it's still not quite square on to the sight body, which you can see in the second photo. So at least a couple more prototypes to go to fix those up and make it easier to build.
I also realized (duh!) that there will have to be a separate version for lefty shooters, since it's difficult to see through their heads...
