This is a very simple but extremely handy accessory for manual fine focusing of the EF 200mm f/2.8 L lens (also fits other Canon lenses like 100mm f/2 and 50mm f/1.4).
The principle is very simple - a 20cm long lever (M6 screw) is attached to the lens focusing ring using a sewage pipe holder for 80mm PVC pipe.
The pipe holder was sanded and filed for more round "touch" in order not to marr the focusing ring soft rubber surface. The standard Philips M5 screw, used to keep the halves of the holder together, was replaced with slightly longer winged M5 screw which can be tightened without any tools. There is a small piece of yellow plastic attached which keeps the M5 nut in it's socket - I hate losing these small nuts in the grass every time :-)
In the field, the lens focusing ring is first set to infinity (lens set to manual) and the lever is attached to point downwards. A small tap to the lever moves the focusing ring a very tiny amount - it is roughly 6-10x more precise than directly manipulating the focusing ring.
It is a very handy accessory, quite useful with camera in life-view mode (my 40D) and in particular with Bahtinov mask - field tested while imaging comet Lulin.
Highly recommended and it takes at most 2 hours in the workshop.