Spotted this page and decided to try this unusual variable capacitor idea, I had an 20cm heat sink laying around so splited it in two to see the results.
We can "juice" just a little bit more getting the plates closer.
Now fully open....
There's just a small problem, it takes some space and the mechanical parts are not easy to make but should be easier to build than a classical unit. Regarding the price, this "unit" cost'd me 7 Eur some years ago. Now the 20cm heat sink costs 11 Eur (dam inflation), total cost for a typical 400pF for an ATU should be around 22 euros, plus the hardware for turning.... sometimes we can get cheaper caps in ham swaps.
One big advantage of this design is that there are no wiper connections so lower losses and no risk of bad connections.
As far as breakdown voltage, considering 30Kv/cm in normal conditions we can assume that at least 1mm separation gives 3Kv... but on a safe side I would consider just 1Kv...
....I think in the future I'm going to rebuilt my antenna tuner/matcher.