10mm thick plates are good. do not go too deep. I'd say about 4mm on each side should give you awesome flow-rate and yet you will have turbulence and surface area to spare.
depending on the size of your block, but 4mm diameter is a good overall size. Leave at least 3mm on the sides, for sealing the block up. and before you seal it... lap the surfaces, so they fit snugly on one another. then just a minute amount of sealant, in between, and you would be surprised at the bond being formed between the plates....
of coarse, make sure you do not have any oil or dirt on the surfaces....
