I'm trying to write some $\TeX$ conversion for a tensor with an upper and a lower index, but I would like the lower index to be displaced to the right by the width of the upper index. In (La)TeX I would write this as W^I_{\hphantom{I} J} ($W^I_{\hphantom{I} J}$). I tried to use the Invisible Mathematica command, which seems to be the counterpart of $\TeX$'s \phantom, but instead of getting a space I get the upper index repeated as a lower index.

Here's the code I'm using now, which doesn't quite work:

w /: MakeBoxes[w[i_,j_],fmt_:StandardForm] := (*BUGGY*)
SubsuperscriptBox[MakeBoxes[W,fmt], RowBox[{MakeBoxes[Invisible[i],fmt], MakeBoxes[j,fmt]}], MakeBoxes[i,fmt]]

This does work in the notebook front-end, but not when converting to (La)TeX. What is the best way to achieve the result I'm after?