I did similar thing .
What pump/ hose size??
What dimensions on impingement slot? I made mine 3mmwide. I think that I made it a bit wide.
Dimensions on slots ect..
Here is mine
Mine is 6 fins .9-1.2mm thick and aprox 2mm slots.
10mm thick block and aprox 8mm deep slots.
Heto 3800 running ½ inch tube. Through motorcycle radiator with a 120mm panaflow @ 7volts. No load (ie machine is file server/firewall) after 1 week (30 deg c idle--full load 37deg)