I'm sorry Robert, I didn't see your earlier request.
I've done some fairly decent representations of Bunkers AND tents in the past, and will be more than happy to share with you my techniques.
The tissue can be formed easily when wet, and once painted, dries hard and looks very real.
Here's a bunker with canvas top I done on a dio.
Of course, I'm sure there are AM sets available (for a price), but this construction is fairly straight forward (although making all the sandbags is tedius) and Cheap!
Hope this helps a little.
Take care,
Frank