My immediate thought was to wrap them in some sort of diamondoid shell through some sort of mechanosynthesis process, but maybe a carbon nanotube would be stronger?
couple it with some double walled carbon nanotubes and you can get upto 700 GPa
I wonder if one could just stuff the tubes with hydrogen, then lanthanum powder, then just let them expand? They might auto-heat from the pressure, and assemble themselves, if you just unfroze them.