The powers that be are looking at it the wrong way. SpaceX essentially want to build a space shuttle to Mars or a bigger lunar lander.

Columbus didn't row to America. He took 3 ships and a lot of food. We need something approaching the Ares III from The Martian, a rotating central drum providing 0.5g of force over a 9-month journey to acclimatise to Martian gravity and a central nuclear reactor that can power the ship and run a magnetic field to block charged particles.

This would leave 2 things to worry about: Gamma rays and neutrons. Only 400mm of lead is needed to block gamma rays but neutrons are more problematic. It is feasible a positional shield that is always pointing at the sun could provide a decent enough 'shadow' to protect a ship. This could be a lead shield with an ice underside. This wouldn't melt in a vaccuum but would provide adequate neutron protection.