Generally, you need a drill, some threaded bungs, a welder, some 90 degree fittings, some clear fuel hose and a couple of clamps. ¹⁾
From there, it is all up to you where it goes, but you do want to put it in a spot that actually shows you usable fuel.
So putting the sight level on top of the tank won't do you much good.
²⁾

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