Is anything going to cover the insulation after it is installed? Is it on the bottom side of the rafter?
A few pieces of cheap wood strapping can be used to hold it in place.
If your intent is to remove the insulation seasonally ...... just shoot a few wood screws in with a fender washer. We are not talking about anything heavy.
I considered the wood strips, but was looking for something a little easier/faster, to put it up.