The lightest and most efficient shield is water. It's low atomic numbers recoil and sap the energy out of the cosmic rays. Rock shields make secondary radiation that can't be shielded. We have the equivalent of 40 feet of water above our heads, mass wise. A sphere of 80 feet diameter would have a mass of 8,600 tons. We will need to devise HUGE engines to get that much mass up to speed.
OTOH, if we accelerated just the living quarters towards an asteroid, in a relatively short time we could land, build a shield, take off, and not lose speed. No massive engines needed. The asteroid itself having been at solar orbital speed before we got there. Then we just tweak it.