The mass of a Voyager RTG putting out 158W is about 38 kg (84lb). (It looks like Voyager used 3 of these RTGs.) Seems like they would be too heavy for your pu

The mass of a Voyager RTG putting out 158W is about 38 kg (84lb). (It looks like Voyager used 3 of these RTGs.) Seems like they would be too heavy for your purpose. It seems the Galileo RTGs were more powerful: 300 W in 56 kg (123 lb). This is still pretty heavy but maybe getting in there.