I am right handed, but left eye dominant. I could not hit the ocean from a rowboat with both eyes open. So I shot a shotgun poorly for decades with my left eye closed. And couldn't hit anything at all left handed.
A trapshooter suggested I try a Uni-Dot. This is a fiberoptic rod in a tube that sits on top of the rib. The reason for the tube is that the end of the rod is recessed back in it. If you shoot right handed, your right eye can see it, but the tube blocks the rod from your left eye. Now you can keep both eyes open for 3D vision. It is very important that you do not look directly at the bright dot. You must focus on the target and, with peripheral vision, bring the dot to the target. This is how it should be done with a regular bead anyway.
It took me two months to get used to it, but then I ran my first 25 and 50 straight in trap.
The specific model I'm using is the P210, which has a 2mm dot. I think the 3mm & 4mm dots are too big for any kiind of precise shooting, and they are no brighter than the 2mm rod. The P210 has interchangable rods (red and green, but at one time yellow). Even if you don't care about different colors it's easier to replace a rod should you somehow break it. All of my ribbed shotgun barrels have Uni-Dots on them. I have 10 of the P210 and one P200 model. Mine are the adhesive model but these are also made in a snap-on style. I would not have shelled out that kind of money for that many if they didn't work. I also took up trapshooting and sporting clays, now that I can actually hit the clays, as well as upland bird hunting.
Uni-Dot®
http://www.uni-dot.com/pdf/Unidot.pdf (catalog with prices, which have actually gone down over the years)