I am working my way up to doing the same thing, and would recommend using 2" foamboard, even doubled up in thickness if you want the extra R-value. In the past, I have had very good luck using high-quality duct tape to hold Dow foamboard together semi-permanently, which allows you to dispense with framing something up out of wood and then having thermal bridges.
Of course, I've never tried taping something in an attic with the summer heat issue - so an adhesive might be a good idea, too.
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