I don't have the answer (sorry!) but simply more questions!
How will you insulate the raised planter in the winter? How much soil will you use?
I'm trying to think of using the same idea here, but I'm lucky my hops survive in the ground year to year, I think, since it's so cold here. I mulch, and they seem ok so far. But I don't think having a raised box would work for me here. I'll be curious to see what others do!