This is a small HHO generator used for filling balloons with HHO gas.
I use 20 gauge stainless steel for the plates which rounds out to be about 1mm thick.
I cut and grind them until they fit. Then, I solder some wires to the plates. For the voltage, I ran at 30 DC, but only for a little bit to prevent melting! If ran at a lower voltage, it will be safer (which makes sense). Be safe! Go blow up some balloons! :P
Have fun with the print and build :)