I'm not sure a cooler is even needed with a manual box. Because the auto fluid is used to actively translate the engine rotation into the gearbox, it's understandable that it will heat up and particularly so when towing, but in a manual, the stress is all taken up by the cogs.
There might be a small amount of shearing on the oil molecules in the gear faces but that should never amount to enough to worry about.
If you really want to do it, why not get the pump section from an auto box, replace the pressure plate and clutch with a torque converter, fit a TCM unit to the side of the box and wire that into the ECU (ports 57, 58, 87, 88, 121 and 125 on the ECU). You may need to add some clearance in the floor pan (above the gearbox) for the TCC solenoid and clutch assembly but if you're happy with a home-made tiptronic who cares about a bump in the floor? The whole assembly is about 150mm in size, but the manual box is 25mm lower than the auto, so you only need to raise the floorpan in the cabin by 125mm to clear the solenoid assembly.