you can build it with some poster-board and tape. I would use the 20 diameter rule - meaning the length of the shroud should be 20x the diameter of your radiator. You'll have to use the hydraulic diameter to get a measurement of the radiator diameter, since it is not a circle. Wikipedia will tell you how to measure that. It is easy. Then, from fluid dynamics, fully developed flow occurs 10 diameters away. So, that is where I would put my thermocouple (as close to the middle of the duct as possible). At that point all mixing will be done and the temperature there will be a good estimate of the average temperature coming out of the backside of the radiator.