The actual initiation is the easy part and would have been easy in the 70s, exploding wires that are subjected to a number of kilo Ameperes by some kind of avalanche, solid state switch.
[1] https://nuclearweaponarchive.org/India/IndiaSmiling.html
Curious... is timing actually that big of a deal when you've already got critical mass? What would happen if you just dropped one half critical mass from 1m above to the other half?
Even though OP had their research taken away, I wonder how long it will be before you could simply ask GPT-3 how to make a nuke for under $2k
https://en.wikipedia.org/wiki/2006_North_Korean_nuclear_test...
They had a yield of less than a kiloton. Obviously way greater than conventional explosives but it should have been an order of magnitude more powerful.
In order for the explosives to form that crucial explosive lense you need detonation across numerous points on the outside simultaneously so you need a detonator that's extremely consistent on timing so a simple resistive detonator with some primary explosives packed around a filament is not going to be effective. The method of choice is an exploding bridgewire design. Basically you have a thin wire and you pass enough current through it to vaporize the wire so quickly that the wire itself generates the initial shockwave as it gets turned into plasma. The rest of the explosive lense is just two different types of explosive with different propagation velocities. The two are combined with a parabolic (or hyperbolic, I forget off the top of my head) face between the two layers so that the explosion that started from a single point goes from convex to concave so it hits the outside of the pit all at the same time.
If you simply dropped a half-critical mass onto another and it was prompt critical then yes, it would explode, but with not nearly as much energy being released as a proper nuclear weapon.