Arching guide for the back

I can now start shaping the outside of the back. The order of operations seems to vary a lot from maker to maker. Some now establish the edge thickness and do the purfling before starting on the arching, others start with the arching and do the purfling in the end. In any case I thought now would be a good time to make at least the long arching guide for the back. Last time I had done it from thin plywood. This time I will try thin aluminium sheet (0.8 mm). Mainly to be certain it keeps its shape.