Thin walled pipe can be stiffened by stuffing it. Anything that stops it going oval will stop it bending because the main way it bends is by going oval.
I tend to use prunings - they don't have to be in one piece - if the thickest part of each piece fits the pipe it will still have a dramatic effect - and wide pieces take longer to rot.
This method also allows you to change the curve - if you don't stuff the middle bit it will bend to a more pointy roof line - very good for shedding snow - and that allows you to use less and/or lighter materials - but don't go too far - because (on the other hand) the steeper parts of the curve always catch the most wind force.
Cheers.