I finally went with this GPU cooling setup, the previous one was using too much space at the back of the GPU card and thus would have interfered with the PCIe riser or even touched the mobo:
Because of the weight of the copper block, as you can see I played it safe and add a backplate at the back of the card to help supporting the block. A modded CPU bracket did the trick!
The hardest task here was to find the right springs to put in between the backplate and the card, not too big, not too strong but enough of both.
There is a now short 1mm gap above the Direct Power thing, everything fine!
After several tries I have been able to adjust the copper block so that it touches the die properly when attached to the GPU cards. The contact could be improved on on the top left corner of the die, but all in all this time the pressure test looks good to me, unlike the CPU side.
Because of the relatively big size of the copper block, I had in mind to save some of my remaining pressure papers and instead check how the contact was with the thermal paste left on the aluminium heatsink.
However, it turned out that once in contact, there was no way to separate the copper block from the heatsink! Pulling on the heatsink only acted on the backplate springs, I would not have bet on the thermal paste being that sticky!
Anyhow, I concluded that there was a good contact there! 
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Minus the vapour chambers, the prototype of the CoolerCase mini version is now ready!
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I will next check the cooling capacities on the GPU, hopefully getting better results than what I got with the CPU.
