I replaced my windows with foam-sandwich fibreglass panels which I made for the purpose, which is similar in principle to the approach described in your link. The only difference really is that my panels were laminated using wet fibreglass so became single homogenous units, rather than being three dry sheets of material held together with screws and glue. Mine are probably thinner as well - my panels are the same thickness as the original double-glazed window units, so directly replace them - but a thicker panel which matched the thickness of the steel tube of the window opening itself would have worked equally well, and would have a greater insulation value.
Here's one of my panels (with a 'bubble' in the middle to provide a few more inches of interior space in the bus):
There's a bit more info on my page in the projects section: http://www.busconversions.com/bbs/index.php?topic=21255.0