You may not need to. Follow the advice given in that link, to run the mesh through PM2IM with Strip Textures set. Right-click View Errors and Warnings while the asset is still open for edit; if the error is still present, you can simply revert the asset and effectively "undo" the repair.
Sporbust's assets are unusual in that "both types" of major texture errors (apart from not being a power of 2) are present in most of them. In other words, errors in which textures are legitimately missing and also in which old textures weren't used (and thus could be stripped out).
P.S. ...and if you do need to create one, just make a solid BMP file in Paint or Gimp that's a power of 2 - I recommend 16x16. Just make sure the *.texture.txt file points to it.
