You might think about making each drive-cooling sideplate out of a plate of copper, up to, say, a quarter-inch thick. Mark where you need to put drive-mounting holes, then solder a "serpentine" of thin-wall copper tubing, staying away from the reserved-for-holes area. Should be easier than constructing a water-tight "box" on each side, or drilling galleries through a thicker plate. If you have flow problems (and you probably will, unless you build a pair of "boxes") just use a pair of 'Y' connectors and split your flow between the two plates.
Many radiator shops can do the bending and soldering. HVAC shops have the tubing.
Rather than automotive radiators, maybe look at really big automotive heater cores? Check out the Heatercore Database
They go up to about 11x6 (bigger for commercial trucks).
Also - for moving air quietly through automotive stuff, look at stacking
axial fans - or running some kind of impeller - like maybe this 24VDC centrifugal fan
- dunno if it'll start at 12V...
Also check out the ProCooling water cooling forum
. There's a lot of water cooling know-how there.