For cutting a deep straight line of some length like this, put a long heel on the graver.
That will help guide it in the cut each time an new (repetition) pass is made. It'll
have less tendency to wander side to side and widen out the slot.
It won't do for much else as the long heel makes it near useless for turning curves but as a specialty tool for
things like this and cutting for straight line inlay it works nicely.
Cut to depth, then under cut the sides and the ends as already mentioned.
Easy does it,,it doesn't take much to grab the softer metal and a larger undercut doesn't mean a better hold on the inlayed material.
Either file the base concave slightly or as I do on such inlays, I arch (bend) the piece of metal itself. You want it to fit snugly in between the 2 ends of the cut which you have undercut. Those undercuts on the ends of the line are simply done with the same chisel you cut the line with. Just let it dead-end squarely into the end of the finished cut, then a few more gentle taps and it'll spike an undercut wedge shape into the metal as well as raise a small burr for you.
The sides can most likely be undercut with the same chisel run along the bottom edge on both sides if the face angle is acute enough and/or the cut wide enough to allow you to get in there with it.
Place the inlay in place and tap it gently down in. The concave shape or arch will flatten out onto the bottom of the cut and the ends will enter the undercuts at each end if you've prepared the cut and inlay well.
You can't get too much of a raised inlay (like this sight) to upset and enter the side undercuts w/o flattening the thing but a tiny bit will if you anneal the silver first.
Tap the raised burr from the undercutting down flat all the way around the inlay.
For an added hold, just before setting the blade into place, tin the bottom edge with soft solder and put a touch of flux down in the groove.
Set the blade as above, check for square and then heat till you see any visible solder melt. Don't over heat.
You shouldn't have much if any excess solder on the blade or bbl with just the light tinning done to the blade.
You can swipe the parts with a pencil lead before heating if you want to avoid any excess from sticking to the bbl or visible blade. Makes any cleanup easier if you do have some spill out.
A simple graver suitable for this can be made from drill rod.
A friend used to make some of his gravers from small 'allen' wrenches , cut the L portion off, shaped, put into wooden handles and used w/o further heat treat on brass, silver, copper and some soft steel.
(Some of the allen wrenches I've seen lately probably wouldn't cut linoleum!)
Maybe they can be further hardened too, I do not know. Most M/L bbl steel seems pretty soft though
