Even if sources have been lying to me, which is certainly possible, I believe I understand enough to determine cross sections by experiment and from that to determine critical masses; for isotopic enrichment I know about the calutron, which is meh but works and can be designed from scratch with things I know (though caveat have not memorised, just that I know the keywords "proton mass" and "Lorentz force" and what to use them for); for trigger, I would pick a gun-type design rather than implosion, again this is meh but works and is easy.

A few tens of millions of USD mostly spent on electricity, a surprisingly large quantity of natural uranium (because the interesting isotope is a very small percentage), and a few years, and I expect most people on this forum could make a Little Boy type bomb.