Turn your sight upside down and put on hard surface like the anvil on the back of your vise. Peen the bevel edges inward lightly from the underside, test fit and repeat. The metal will expand ever so slightly, you can get a good tight fit this way if you take your time, don't get overzealous on the peening or you will have to file some off to even get the sight into your dovetail.
Always file the sight if you must, never the dovetail.