Built from low cost and easily found materials.
A standard diy arc welder (transformer type - heavy) produces about 40 volts AC.
I connected this to a bridge rectifier to convert it to DC. These are availabe in most electonics stores or Ebay for about £10 or £5 if you are willing to wait for it to come from China!
Make sure you get one with a heat sink as it will be working hard with 2 or 3 cells running off it.
With the welder on its lowest setting I get 36v DC from my bridge.
Connected across a single cell I got a lot off gas but also some intenal sparking (very bad with HHO gas present) and heat so did not risk using this set up.
Split across the 2 cells and a stainless coil in series provides about 12v to each just as a vehicle battery would.
The 12v per cell seems stable and runs well for long periods with little heat generation.
The construction of my cells, bubbler and 'torch' are mostly from scraps or standard diy materials.
Welder - transformer type - about £60 but most diyers have one lying around
Bridge rectifier - about £15 - had to buy two as I blew the first one which had no heat sink
Double pole switch - £3
Jars - 3 for £5 in a local bargain store
8mm Copper tube - left over from an oil tank installation
Thin stainless tube cut from a ball inflator
KOH - drain cleaner crystals - £1 from a local hardware store
T-pieces / straight connectors & plastic tubing - fish tank fittings from Ebay - about £6
Bubbler - herb jar with a rubber tube siliconed into its neck and an old broken drill chuck set on top
Stainless wire taken from old rigging cables
Stainless mesh plates cut from an insect screen
Already had the welder so reckon the additional parts came to less than £40