I would assume it would depend on the quality of manufacturing.
A small amount of highly pure uranium, well-formed, with a well-formed and powerful surrounding conventional explosive could probably be done. But, fortunately, the smaller, the more difficult, and the larger the technological support the society must have.
IIRC, the US notified all the potential nations that might supply (i.e. "lose") a suitcase nuke (or "lose" more conventional nukes) that should one be used against the US, the US could tell this, and there would be hell to pay...so they'd better let the US help them track these things somehow.