It seems among the wide range of Wolfram symbols, one is missing. This code

Graphics[{
      Line[{{0, 0}, {1, 0}}],
      Line[{{0, 0}, {0, 1}}],
      Text[Style["\[Square]", FontSize -> 18], {0.014, 0.014}]
  }]

will make the usual "right triangle box" where two lines meet. But it needs to be generalized to be adaptable to lines that are orthogonal to one another but not parallel to Cartesian Coordinate axes as my simple example is. Ideally, it would accommodate differ aspect ratios, line style, 3D etc. all the choices one would have under normal visualization conditions

Here is something a little more robust but clunky

u = {7, 8, 9};
v = {3, 6, 9};
oVecs = Orthogonalize[{u, v}];
twoLines = 
  Graphics3D[{Thick, Red, Arrow[{{0, 0, 0}, oVecs[[1]]}], Blue, 
    Arrow[{{0, 0, 0}, oVecs[[2]]}]}, Axes -> True];
rightAngleSquare = Polygon[{{0, 0, 0},
                                0.05 Normalize[oVecs[[1]]],

    0.05 (Normalize[oVecs[[1]]] + Normalize[oVecs[[2]]]),
                                0.05 Normalize[oVecs[[2]]]}];
rightAngleSymbol = Graphics3D[{
        EdgeForm[{Thick, Black}],
        FaceForm[Yellow],
        Opacity[0.01],
        rightAngleSquare
    }];
Show[twoLines, rightAngleSymbol]