M10 bolts would be a bit big? For my gpu block I only have space for 2 M3 bolts, which will be enough, as it doesnt leak without bolts.
A dremel can stil make good blocks with a good design
Taps are expensive (about £25 each in UK iirc) so Im not using them. For the barbs plastic ones are best so that they wont crack the top if they get overtightened. Im going to glue the barbs to the top, instead of tapping holes and using ptfe to stop them leaking.
Also in plastic you might not even need taps. For the mounting holes Ive done I used a 2.5mm bit, for 2.9mm screw. In the plastic the screw bites into the side and taps its own thread. Just tryed this with the silver and it was easy, just use the screw like a tap, 1/2 turn forward, 1/4 back...