I use 3/16" (~4.75mm) thick copper plate for the tops of my blocks. I wouldn't recommend much thinner. If you want to seal barbs well, grab some Loctite 567 liquid teflon thread sealant. Rated for ~10,000 PSI, which I find kinda hard to believe, but hey, even if they exaggerated pressure by 100-fold, it's still rated for almost 100x more pressure than the seals will ever see. I use BSP threads, which allows for about 3 thread turns in the 3/16" plate for the barbs to fit. Using the Loctite 567, I reckon you could get a leak-proof (as far as water-cooling is concerned) seal in one thread if you really wanted to push it.
As with most things in life, if you find the right tool for the job, then it opens a whole new world of "what's possible".
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