anyone thinks this would be a bad solution for a custom watercooling block?

of course the last row of chips will be the hottest this way, but you could switch exit1 with 4, so the hot output stream will be next to the cold input, therefore cooling more equal. Plate is 25mm (could go down to 20, maybe less) and holes are 10mm
Of course it can be scaled down, because 20watt don't need that much waterflow. I'm just trying to find a good working and easy to make cooler
