Have you tried Zen? After decades of switching dominate eye from right to left at will, I can pretty much make my eyes do anything except get younger. I shoot all firearms with my right eye but my bow with my left eye. It is a very mental thing with me and I can switch which eye I am seeing things with very easily. What you might do is try some exercises such as holding a pencil out at arms lenght while wathcing TV and use it as a sighting point and try to only "see" the area right around the tip and not the stuff out to the sides.
Now that I have dispensed the 60's Haight-Ashbury advice here is something you might think about.
Working off of the same principles as the iron sight aperture, if a person got some of those geeky flip up and down sunglasses and took off the left lens and then drilled a 1/8 inch hole in the right one at exactly the correct place it would give you "tunnel vision". You can run a trail test with just a piece of paper with a hole in it to see if it is worth the effort of makeing the glasses.