panes.mdl represents a series of glass panels, originally inspired by those found on bus shelters. The visible part of the model is just the silvered edges of the glass panes. On engines without transparency, having these portions opaque still produces a reasonable effect. Since the main face of the panel is not rendered at all, you can still see through the window on these engines.
The model is improved by applying alpha to it, making these silvered edges semi-transparent. I recommend about 50% transparency. Because the model works fine without alpha support, you don’t need to detect whether the engine supports it to use the model. Just place it in map, apply the alpha value, and in engines which don’t have alpha support it falls back to the opaque effect.
In size terms, the model is 8 units thick and spans 256 units across by 128 units tall. I recommend building a well defined window frame about the entire window pane. You can hide the tops of the panes inside brushes to create shorter windows. Having recesses for the edges of the panes to enter helps cement the illusion of solid panes of glass between the visible ends.