My usual way is to fashion some sort of hook from heavy wire and go under the hood to pull/release the spring from under the dummy radiator cap. If you're getting one from a salvage yard, maybe you can just pull the retaining clip and get the whole dummy-cap with the star attached. Occasionally, when I'm parked in risky areas, I'll pull the clip and lock the whole cap & star assembly in the trunk.

Happy Motoring, Mark

__________________
DrDKW