Most common off the shelf commercial sights like this not so much a spring, but they flex enough in the soft state that the steel itself does what is needed. That being hold the elevator down against the bbl.
If you pull up on a lot of these, you can bend them easily enough and leave the tang section pointing upwards.
Bending them back is just like bending a soft piece of thin steel and leaves a crinkle in the thin tang where it bent.
Some seem to be made as a spring. Some of the MArbles mfg are very tough and spring like.
But most any steel will have some flex range to it. Wether it's enough to meet the needs of this application is the question.
If I were going about making one from scratch, I would want the thing to be a real spring.
1075 annealed steel works rather easily. The entire sight could be made as a flat piece probably out of 3/16thick stock.
The blade end could then be bent upwards when finished with the above work by heating red.
Then finish any shaping.
Re-polish, harden, then draw the temper to make it a spring.
I use a lead bath at 735F for tempering. Never failed yet.
Imparts a nice temper blue color if you want to leave it as-is.
Smoke the part with soot before the lead bath to avoid the lead sticking to the part. OA torch acetylene gas flame works quick.
You can still cut,file, etc the finished spring if you really need to. Sharp files will do that for you. Polish carefully afterwards.
A thin tapered tang section as a spring will make lifting the sight for elevator adj easy. But still always give secure downward pressure against the bbl.
