Texture mapping of slanting surfaces

[Hans_Petter]

I've created an element that's basically a cage that encloses another structure on a roof top. I've used transparency for the grid of beams and crossbars.

The structure has rounded end sections and the top surface is smaller than the bottom surfaces--this means slanting surfaces and trapezoid / trapezium surfaces at the corners. How would you go about texture mapping this? As the attached image shows the rounded corner sections don't match up with the rest.