I like using bamboo for trellises. It's more attractive than wood, and even stronger. I buy substantial uprights, at least 1" thick, and drill small holes to put short lengths of wire through to attach the horizontal pieces. Very verstile and lightweight.

Um, you said: "without having any wooden support blocking the window from opening". Does your window swing out?

My other suggestion would be to use galvanized wire (doesn't rust) attached to screwheads or eyehooks, instead of using clear netting. The netting sounds like a good idea, though. Maybe you could find a stronger net?

What about eye hooks and a really strong (50 pound test or more) fishing line?