You need to compress not the solid chunks of metal, but the gaseous / plasmatic plutonium after a few microseconds past the critical point. The problem is that it emits neutrons too quickly, and the thermal runaway of the critical mass happens too quickly. If you do not hold the plutonium together, it will just fly in all directions before enough of it has reacted.

No static casing us string enough; a dynamic caring made of highly compressed results of a conventional explosion works.