A couple of days transiting the Van Allen belt shouldn't present much of a mass issue, since Gemini showed you could shove two astronauts in a phone booth for a week. About 6 inches of polyethylene would stop most of it (99%) and with California bans on plastic shopping bags, there should be enough of it freed up for deep space applications.
Almost any proposal for near-term deep space needs to include something like an upsized, shielded Soyuz orbital module with its own life support, power, sleeping arrangements, and bathroom, which due to its mass and sophistication should be reused across many missions. That means it needs one or more docking adapters and perhaps its own minimal RCS system for station keeping between missions. The ideal way to test the living arrangements would be to attach it to the ISS for several months, perhaps as a block I non-shielded to be followed by a block II shielded version that includes layout and equipment improvements based on experience with the block I. Unfortunately there's no current funding available for such a module because everything is allocated to the SLS and Webb.