MacGyver Ventilation

Just take down all your MacGyver posters off the wall, Roll them up to what ever diameter you need and tape them together.

Now you will have real deal MacGyver duct

Dryer hose is under $10.....its not expensive...why do you insist on making your own? It would take a long time to do this right..... How little $$ is your time worth?
Yea you could make your own but you might come home one day and find that your homemade duct collapsed on itself, choked your fan up, causing your bulbs to overheat and pop, and your plants will be dead. Dont cause a fire and make other growers look bad.
All this risk to save $10.....Sounds like you are making it harder for yourself my friend.

if you're really doing this, cardboard works great as ducting. you can fold it into any shape. I have seen legit HVAC guys use cardboard in a pinch to make a (temporary) fitting. but I suppose you could use it in a permanent location as long as there isn't TOO much heat.