Steam Bending

I am building a Marble Run Game for my daughter that needs a spiral for then
marble to go around and down. The plans called for a spiral cut out of 1/4 in plywood and anchored to a form. It was just forced into the form. I broke several of them and came to the conclusion that that this system would not work.

I cut a spiral out of 1/4 in balsa and tried steaming it for 15 minutes. When I tried to fit this in the form it broke as well. After much thought and reading/ studying, I think that steady pressure and steaming will make it work.

Any assistance or expertise in this would be appreciated. I am new at steaming.

 

What is the smallest radius? I've built F5 mandolins and I find 1/8" maple bends pretty easily over a hot pipe. I never tried 1/4" but you could double it up.