Groth, have you tried a Honda heater core? Rather than one wide flat tube, they use three at any one point. Cut it into a one third piece and a two thirds piece, so you'd have two with the same surface area but one twice as thick... Perfect for this test. You'd only need to seal the top and bottom that way.
So flow rate is at least 7.5LPM, probably much more? (Asking because the thermal resistance of the block is higher at lower flow rates)