Are you sure you want to use wood? I'm definitely not an expert on wood but it seems to me that with all the watering that cannas require would only make the wood rot rapidly. The canna rhizomes would get large and strong enough to push right out of the contained area.

But in answer to your question, I would think that no more than about a foot deep as they're a fairly shallow planted plant. How large of an area are you talking about?

A thought that I just had was to plant your cannas into large planters and just place them into the bed/beds. You could either sink them or not. If you use large decorative planters you can just place them among the foliage and flowers of other plants and the planters can be garden accents.

Jenna