One option is to inject some Super Glue under the insert and press it down into position.
You can make an excellent insert with epoxy glue and red Testor's Model paint (solvent type).
Use a solvent like Acetone or lacquer thinner to degrease the sight.
Make up two "dams" from small, flat pieces of metal or plastic.
Apply a coat of wax to the faces of the dams, then clamp them to the sides of the sight blade with a small pair of Vise-Grips to form a mold around the dovetail.
The wax prevents the epoxy from sticking to the "dams".
Mix up some epoxy glue, something that has a longer working time than the 5 minute stuff. The 1 to 2 hour type is ideal.
After mixing the glue, put a SMALL drop of Testor's solvent base model paint in the glue and mix it thoroughly.
You need only a small amount, and the less you use the stronger the insert will be. If you use too much the insert will be weak and crumbly.
Experiment to find the right mix. Use just enough to color the glue properly.
Use a toothpick or small screwdriver to put a drop of the epoxy-paint mix in the dovetail.
You want exactly enough to fill the cut to the top of the sight, level with it but no higher.
Brace the sight, (or the whole gun if it's on the barrel) so the top of the sight is level.
This will allow the epoxy to settle level and not run out.
After the leftover epoxy is like a hard rubber, remove the mold and use a brass or plastic "knife" to clean up any excess epoxy.
Allow to fully cure for 24 hours in a warm place, and you're good to go.
An additional step you can do for an even more durable insert is to put S&W holes in the dovetail.
S&W does, or did drill two small holes in the bottom of the dovetail in the sight.
When filling the dovetail with the epoxy-paint mix, use a needle or toothpick to pack epoxy into the shallow holes.
These form "locks" that prevent the insert from coming loose or moving sideways out of the blade.
This home method works just as well as the commercial sets, and is considerably cheaper.
It's not really all that hard to do this with a little practice, and you can save some money.