I'm sure a physicist can get the details of this better than I can, but I'm pretty sure it's sufficient to create a subcritical, hollow sphere of fissile materi

I'm sure a physicist can get the details of this better than I can, but I'm pretty sure it's sufficient to create a subcritical, hollow sphere of fissile material and then slam a matching sphere of fissile material into it very fast. The "very fast" portion could be accomplished with a steel pipe and explosives. The manufacturing would certainly be within the capability of a reasonably skilled group of insurgents/freedom fighters. The big question is where they'd get the fissile material.