Hey Max,
If you create a new Material, select it to be FSX, then scroll down the Material to Framebuffer Blend, and click the button on the right marked 'Set Default Transparent'.
Then, when creating your graphics, you will need to add a Alpha channel to your Diffuse and your Specular bitmaps/DDS graphics.
If you go too dark or too light on your Alpha channels, the glass will either be invisible or solid opaque. Thus, the Alpha channels will control Opacity amount.
Its one heck of a difficult chore to get the colors and Alphas correct.
There is a base set of BMP's in the Gmax FSX adapter tutorial I did at FFDS that will show you the Alpha channel amounts. Just open them in DXTBmp and you should then be able to export the Alpha to your graphics program and note the amount of RGB amounts, such as 112, 112, 112, etc.
The RGB's that I used on the Epic LT are;
Plexi = Your preference, I used a mottled goldish material color to resemble gold tinting
Plexi Alpha = 151,151,151
Plexi Specular Top Color = 0,0,0
Plexi Specular Alpha = 255,255,255
It took me many days to get the 'recipe' the way I wanted it. I spent one entire day trying to find the right amounts of Alpha so that the part was back to transparent again, as I didnt back things up. Very frustrating.
I very much miss the basics of FS9 model creation, especially in Materials creation.
Bill