I remember that someone heated Jobo tank with a nearby tungsten bulb. The black plastic absorbs light and heat well. The correct wattage and placement is crucial, but once you find it, you may be able to slow down the cooldown considerably, lessening the effect of variability. On the other hand, while you try to eliminate some variables with this kind of method, you get new ones such as the placement of the lamp and how long before you put it on etc. If you want to try it, as a starting point I would use a 75W reflector bulb at 30 cm distance put on a minute before pouring the FD in....