3/8 seems about it indeed. i got a 1/2 npt and that is a bit much for a 1/2 inside diameter barb.
so 3/8 should be just about perfect.
my top half out of the crappy brass is ready, tomorrow i'm gonna pick up the 5m of copper for 80€ :-)
looks like i won't have too much trouble selling the copper. i expect once (rotor's) block is finished those belgian watercoolers might be a bit more eager to buy the raw copper off me.
here ya go, everything about the block will have been made with the equipment i have.