Have you any idea of the thermal specs of what you're proposing? i only ask as I used to be into PC watercooling years ago and helped with the design of a couple of peltier/TEC blocks. A flat tube would possibly be the worst solution as it has minimal surface area, the only design with less surface would be a circular pipe!
My main concerns are that a flat tube design could suffer from many hotspots partially due to the height of that tube, thermal pockets and laminar flow along the surfaces you actually want to be cooling. In decent waterblocks turbulence is actually quite desirable up until a the point it seriously impedes the water from flowing, a restrictive waterblock is an generally an effective one, making up for the loss in flow with far better thermal characteristics.
At the very least you'd want those in/outlets to be on the top pointing at the board. Is thinner bar available? Just seems way too tall!
Hi tom_o, not really the plan at this point was to over engineer. As for turbulence and surface area I was thinking of stuffing it full with 10mm aluminum wire mesh. Another issue was heat distortion of the tube when the ends and take offs are welded on, the outside needs to be a nice flat surface.