The entire problem of appropriately shielding a space station or vehicle translates to the cost of lifting the mass of the shielding material out of a gravity well (as compared to lifting the much lower mass of comparable but un-shielded systems).

The next-generation space station and Mars vehicles should have double-hull construction. The space between the hulls would be filled with mass lifted from the Moon: a mixture of lunar rock, sand, and dust in proportions calculated to achieve maximum practical density per cubic meter. This would provide the “six feet of concrete” needed for effective shielding.

The space station and Mars vehicles, minus their shielding ballast, could be built on Earth or on a lunar base. They would be launched to an appropriate “parking” location in orbit around the Moon, where they would remain while being filled with the lunar ballast mixture. The ballast would be delivered by a fleet of robotic vehicles that would each make multiple round trips until the task was complete.

There would of course be an additional fuel cost for accelerating the higher mass of the completed space station and Mars vehicles to their working velocities en-route to their respective destinations. However the extreme cost of lifting that mass from Earth, would have been traded off for the much lower cost of lifting the ballast from the Moon. The additional costs of the common infrastructure such as the Moon base and fleet of ballast delivery vehicles, would be spread out over many missions over many decades.

All of this is entirely possible within the scope of existing science, with only some modest and easily-achieved improvements in technology.