A friend of mine uses this unique setup. It is the 100-300 lens with a Raynox 2.2x TC. The upper pic is the setup with the lens at 100mm zoom, 35mm equivalent is 100x2x2.2mm Lower pic show lens at 300mm zoom.
TC1 - black tripod collar for lens
TC2 - white tripod collar for teleconverter
The part in front of the tele is the lens hood that comes with the converter. Zooming slot is a slit which allows the teleconverter to move forward and back free while supporting its weight.
Tripod collars were custom ones suited for the diameter of the lens and TC, sourced from eBay.
Although the Lumix 100-300mm lens have a filter thread diameter of 67mm, the actual diameter of the front objective glass is much smaller, and maybe that's why there is no vignetting at all with the tele. The Raynox is the 2.2x HD type which is large, heavy and more expansive than other smaller ones. It has filter threads of up to 62mm. You will need a good step-down ring.
The weight of this system including the lens, TC, tripod collars, support bar and Manfrotto quick plate total 1542gm, length 35cm excluding the camera body. Effective 35mm equivalent focal length is 440-1320mm.
Pimp your gear! :t: