Ideally, you might not even need that much of a waste heat removal, since D + He-3 is aneutronic fusion, you can directly use the fusion products, high energetic charged alpha particles as a very efficient thrust by using electric fields to guide particle beam out of the ships magnetic nozzle. Then the thrust is provided by work done by fusion energized particles against the electric field, which will gently provide the "damped" momentum transfer to a relatively large surface area of the ship.
D + D -sidereactions which would produce poorly usable neutron radiation and wear the engine, can be minimized by running the burning plasma lean on deuterium, rich on He-3. If additional thrust is needed because of the poor energy/momentum -ratio of very high velocity exhaust, shouldn't any atoms collected from where the ship travels could be used to increase momentum by injecting them in to the exhaust, much like a turbofan jet engine is superior to rocket engine when flying in a dense medium?
With magnetic fields created by superconductors, the whole thing might not need that much of a waste heat removal after all.