I've been having a go at making one for the last few weeks, way more difficult than it would appear. As Jim points out moving the cantilever is hampered by the spring resistance.
My best efforts have been about 10" 3mm Boreland cable bonded into a drilled 5/16UNF screw with the other end bonded into a length of copper pipe. It looks awful but it does work. The other is a Ninja speedo cable with the inner and outer cable bonded to the screw and a star knob handle the other end. That looks brilliant but unfortunately the wind on the outer cover is the wrong way round so it takes a full turn of the handle to take up the slack before the cantilever move upwards, lowering it isn't a problem, the wind is the right way for that. Ideally a similar speedo cable with the cover wind the other way should do the trick but can I find one!
Another observation I have found is that the screw works better when it's end is ground semi spherical, maybe an improvement could be a captive ball bearing of about 4-6mm in the end?
On a side note, it's a shame to see the end of Bedford, from what little I had to do with them they were very helpful