Speaking of cell, or series array, design; a most important consideration that many "new experimenters" with HydrOxy overlook is plate surface area/number of plates.
When creating a "series cell battery" electrolyzer, use the largest sized plates that you can consistent with the space available in your engine compartment. Assure that the plate surface area is large enough in order that current density will be no greater than 1/4 Ampere per square inch at your target current flow level. A series constructed electrolyzer designed to operate at 20 Amperes should have each plate at least 80 square inches if possible. That would be a plate 9"X9" square or an equivalent rectangular area.
If that surface area isn't possible within your available space, then make your plates large enough so that the current density is no more than 1/2 Ampere per square inch. This is a compromise that will result in a shorter lifetime of your plates, however, they should have a service life of at least 5 years.
At the 20 Ampere target current flow a plate designed for a maximum of 1/2 Ampere per square inch would be 40 square inches. That would be a square plate about 6 1/2 inches on each side, or the rectangular equivalent.
If you want the plates in your electrolyzer to perform for many years with the highest possible efficiency it is absolutely necessary to keep the current density per square inch of plate surface to no more than 1/4 Ampere.
I know; we like things small and expect big performance. But when it comes to HydrOxy Electrolyzers, too small is poor performance and a short lifetime.
For a Low Voltage Cell fabricated with commonly connected groups of Anode Plates and commonly connected groups of Cathode Plates, interleaved, the same rule applies.
Assure that the total surface area of your plate groups is sufficient to keep the current density to 1/4 Ampere per square inch for each conductive surface. For example: A Cathode Plate that is between two Anodes, one on each side, would have two conductive surfaces of the same polarity, so the total surface area (includes both sides of the plate) would be calculated accordingly for those plates.
With the low voltage cell you have the option of making your plates larger, or installing more plates, to achieve the desired surface area.
For the sake of keeping cost to a minimum, virtually any grade of stainless steel (new or scrap) may be used in an electrolyzer when the current density at the plates is kept low. There is no need, nor any advantage, in using the most expensive grades of stainless when the plates are properly conditioned and sensibly driven in your electrolytic electrolyzer.