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For my version 1 of the Canon 300 f 2.8 L IS lens, I have for many years used a long multipurpose plate on the foot. My preference and what I have used is the Really Right Stuff MPR-1, which is a 6-inch multi-purpose rail. Wimberley also makes similar long rails. The rounded bottom edges and rounded ends of this rail make it fairly comfortable on your hands when hand holding. Of course I also use the rail to mount on to my Wimberley full gimbal mount, any my style of shooting has me often going between hand held and gimbal support.
As additional information, I have been dealing with Really Right Stuff for over 9 years, with many many purchases of L brackets, plates, rails, ball heads, tripods, a panorama rotator, and a macro rail, and have had nothing but great service from them. On the few occasions where there was an issue with their product or service, they made it right promptly, exactly like you would expect from a well run company.