The higher the current thru the cell (higher current density) the higher the cell voltage. Power efficiency decreases as the current increases if the plate area is kept the same. To keep the power efficiency the same increase the plate area in proportion with the current (keep current density the same).
The smaller the electrode spacing the lower the cell voltage. In practice 3mm electrode spacing is good up to about 10A. At higher currents the electrolyte starts foaming and crawling up the plates (reduces efficiency) and the electrolyzer starts spitting electrolyte foam out. For 10-40A use 5mm-8mm spacing.
Best electrolyte is NaOH (1 part NaOH 4 parts water by weight) or KOH (28% by wt). These give the lowest practical cell voltage.
The best electrode material would be nickel, but nickel plates are very expensive. Nickel plated steel plates would also work. The most practical electrode material is stainless steel. The electrode surface conditioning is very important at minimizing the cell voltage. Best to sand (crosshatch pattern) the electrode plates to create lots of fine sharp points.
Bubbler is absolutely essential to prevent backfires from blowing up the electrolyzer. Bubbling the gas thru a water bath is the only safe way to prevent backfires, provided that the bubbler is strong enough to contain any backfires and that the water level in the bubbler is high enough. Alternatively the bubbler can have a pop-off lid or a rupture disk.
The electrolyzer as shown will not be able to take any pressure without leaking. For pressurized operation use a pressure-proof shell (metallic). If you need to store oxyhydrogen gas for a short period of time, put a large balloon on the bubbler lid. The balloon will store the gas at atmospheric pressure and will not be very dangerous if it explodes due to a backfire. Remember to wear hearing and eye protection.