First off, keep it simple and easy to do.
Cutting PIR to a required angle, or using mineral wool is the easier route.
Personally I would use PIR and Illbruck FM330 (check this, may not be right) closed cell foam to close the gaps. For belt and braces, then tape the joints with a some air tightness tape.
You'll be doing all this while the trusses are open from above and below so should have room to do it. Just a bit fiddly and time consuming, but easy!