Uranium isotope separation can be done with lasers, for example, instead of the masses of Zippe centrifuges that used to be required (or the absolutely enormous air diffusion plants and calutrons the Manhattan Project used).
If that's true, then the question becomes "how to produce a compact, cheap plutonium breeding reactor" + "how to separate out the plutonium". Both without dying, of course.
The conventional explosion can be simulated quite well on laptops now + small models can be tested. Exploding bridgewires are an off the shelf item now and not something Alvarez has to invent first. Do we need a big "beach ball" of large exploding lenses? Probably not. Not if we can simulate the explosion well enough (and we can add reflectors).
So how does The Serious Hacker get access to the explosives? Maybe make them in the garage with an upscale chemistry set?
The neutron generator is probably the easiest part to build these days.
So... the reactor and plutonium extraction are likely the limiting steps for a small but determined group of people. I bet lots and lots of cooling is necessary while the reactor operates. That'll be hard to hide!
The British Royal Navy did secretly run a reactor in the middle of London for decades without anyone really noticing though, but obviously they didn't really have to hide it from the security services which might be harder.
(Only the Swedes and the Finns do the ':' thing when they add an inflection to an abbreviation.)
that's not energy efficient and takes a long time, but it's relatively easy access.
https://www.epj-n.org/articles/epjn/pdf/2016/01/epjn150059.p...
~100g uranium / 117kg adsorbent material / month
a warhead needs about 25kg (enriched) uranium.
so 2,5 months with ~11,2tons of adsorbent material spread over a few square miles of sea should give you the needed 25kg. That still needs to be enriched, but once in operation a system like this generates about 125kg uranium/year. (if you need more or faster just scale the system, plenty of space within the 12 mile zone)
Achieving it with household items would be very special.