I would make the sight body from spring steel, I have 1075 so that’s what I would use for the type of steel. That gives you the option to either heat treat or not, without having to make a second one if you use mild steel first. Although, I would make a spring out of it so I could be sure it has a enough down pressure.

The elevator could be made from mild steel or something like 4140 that could be hardened if you want.
Bob