Whoa. Its hard to imagine you could have enough conventional explosives to compress a dense metal by ~10x (?). You'd need some serious containment to direct that energy inward rather than outward. I suppose I have some reading to do.
There is a story about it. When they first brainstormed the ways to make the bomb, even before Los Alamos, in 1942, one of the several ideas was to use explosives to throw smaller pieces of material together, to make the super-critical mass. This was dismissed as too imprecise, but it was still listed in the April 1943 as one of the possibilities in the Los Alamos Primer, which was the orientation booklet for the scientists joining the project.
One of the scientists, Seth Neddermeyer, fell in love the the idea and talked the bosses into letting him try it. He consulted with the explosives experts in Pittsburgh and started some crude preliminary experiments.
When von Neumann was told about these experiments in October 1943, he immediately pointed out what when the pieces of metal slam together at a high velocity in the center, this creates extremely high pressures. Teller then remembered that at such pressures, iron in the Earth's core becomes slightly compressed. They instantly realized that compression makes the exponent in the chain reaction greater, and that this is a new way to make the bomb. They explained the idea to Oppenheimer, and he pivoted the project to the new method.
This did not work. The material did not assemble into a neat ball, but was just making a mess. But Robert Christy, the guy who was making the calculations for this, realized in September 1944 that the slamming of the pieces together at high velocity was not strictly essential, and that a solid ball of metal could also be compressed by an inward going shock, although not as efficiently. Because this was guaranteed to work, this was chosen as the design for the "Gadget".
Ironically, Seth Neddermeyer, who was instrumental for this to happen, has never accepted that the metal could compress.
April 1943 Robert Serber "Los Alamos Primer" https://upload.wikimedia.org/wikipedia/commons/9/9c/Los_Alam...
Interview with Robert Christy where he recalls the invention of the solid core https://www.youtube.com/watch?v=Ez45QEMI5CA&list=PLVV0r6CmEs...
Accurately casting explosive in odd shapes, without different ingredients separating, and without producing voids when the melt solidified, required developing a whole new technology with careful gradients of temperature in the molds.
They tried lots of different commercial and handmade detonators to find which ones would work most consistently. That took an awful lot of time.
The electronics itself was probably least difficult -- a microsecond was already a very long time for the electronic circuits even in 1945. One could use an off the shelf oscilloscope to see if the detonators worked simultaneously or not. Incidentally, 2/3 of the cables in the famous picture of the "Gadget" are not the detonators, but the simultaneity sensors -- reporting the difference between the earliest and the latest detonation fronts.
Everything was tested extremely extensively. Tremendous resources were spent on testing and test equipment. All in all somewhere between 20000 and 40000 explosive tests were performed at Los Alamos during the project.
It is not often emphasized how much of the work was done in the explosives laboratory in Pittsburgh before passing it on to Los Alamos. They have developed the slow explosive. They also reproduced from the earlier British work and further developed and tested the concept of the lenses, together with many other more advanced things which did not find an immediate application in the bomb. The director of the laboratory, George Kistyakowsky, took over the explosives work at Los Alamos, once the implosion became the main focus of the project.
