maybe an insulated outside of the outside tube?
So no leaks to the extern waterbath?
All action just in between the 2 electrodes?
this would increase the capacitance a lot! and would be great but is a little hard if not a single cell...
in one of my last videos on youtube i use a single cell that i did this i simply constructed a single cell inside an acryilic tube and outside the acrylic i added a foil of copper...
however the thicknes of the acrylic is 3mm so the capacitance is virtually zero although it exist... to get a better situation the thickness should be something like at least 0.5mm
another strategy and i think that is the one that worked for me most to view this effect is using a larger cup with more water and thin border...
i confess i never tried to use a single electrode