I suppose one way would be to get an old car battery and remove the plates. Then build up the unit with say, three plates (+-+) in each of the old cell spaces. You would have to be careful to get the external interconnections correct, but you would be sure that each cell was isolated from the next. Any takers ?
Manta
who hasn't been in his shed and done anything useful for two week 'cause it's too damn cold.

Manta old battery casings work well if you can fashion a good sealing lid for it.As far as the 3 plate configured +-+ I feel would run way hot,high on the amps and produce more steam than hydroxy.Remember we're kinda bound by a per plate voltage in regards to efficiency which usually is between 2 and 2.5 volts.using a 14 volt system say on a vehicle would be almost 5 volts per plate and isn't efficient electrolysis.