I'm bumping this to the top for you cause I'd like to know how folks deal with it too. Sounds easy - just sister in another rafter. But there's not much room.
Off the top of my head I'm thinking you could get some extra wiggle room by cutting the new rafter 1 1/2" short at the ridge end and the sliding in a 2x (same height as ridge board) sistered against the ridge board after the rafter is in position. But I don't know if an engineer would approve.
Thanks for the tip jogr. The inspector came by and approved my method.
I wound up double sistering on one side of the existing 4x8 rafter to hold the side wall of my shed dormer. The added 2x's were 4" too short to slide in.. I shortened them on the floor side at the wall plate. I stacked 2x's flat on the floor at the wall plate to rest the added rafter boards on. My floor is made of t&g 2x6 so the weight will spread well.