On Gadget's finger in the tank approach...
While I do have long fingers, I might choose to:
Go through the cap opening with right angle long needle nose pliers inside the tank holding the nut...thinking about where you want the sight glass(es) and have drilled your entry hole. Then insert the sight glass, with an appropriate seal and washer, if needed, to thread into the nut. Notice the sight glasses have six-point bases to allow tightening down. At least that's how I'd do it...I think.
An alternative is to have a .25 aluminum plate welded to the side of the tank, drilling and threading there...or the bung idea John mentioned would surely work as well.
Standing away from the screen now.