@Keet - what an elegant solution

I use this method for every type of glass I need, usually with a flat box I pre-created as glass and just resize it to what I need. It's also an easy way to increase the transparency if you create double or triple pane windows. You do this by setting a lower d value in the mtl file. For multi-pane windows you need a higher transparency because you "see" through multiple glass sheets.

Useful tip for someone who wants to have a hole in the glass for a window-fan or vent: By using a wall you can create glass with a hole in it. You do this by inserting a round window in the wall and exporting only the the wall. The hole created by the window is retained in the export. Edit the export files to change into glass and voila, glass with a hole in it [:))]
(Desperately waiting for the fix to create a round hole in a curved wall...)