IIRC you have to factor in the explosive lenses and various neutron reflectors as well, and to get all the way to the theoretical minimum you basically need literal tons of that other stuff. Backpack/wheeled luggage/shell weight is what’s public, and seems about like about as good as you can get in practice.

There’s exotic choices of fission fuel with a smaller critical mass, but they’re also aggressively radioactive and tend to really really difficult to produce, to the point of impracticality.

Honestly briefcaseness wasn’t the main problem with OP, but it was worth commenting on.