Doubt you need a template. Grab a string a stick a nail in a board. That way you can measure approx where it needs to go and then fine tune it in by moving it around. When you have the true radius you can bring the curtain rod to a metal shop with a tube bending jig and give them the specs.
So, how about this? Up there, zip pole a board over the sides. Locate the centre. From that mark, use a laser to take a few measurements (centre and at the 45's), then construct a template from it. Test.
Alternatively, zip pole a piece of cardboard up there, loosely fitting the area, and scribe a mark a few inches off the wall. Tell them to include it in the template again.
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