Replacement battery is about $20 to $40 depending who you wish to patronise.
The battery should last for about an hour depending on useage pattern
Don't worry ET. Andy has reeeaaaaaally really long showers
Showers or Enemas?
He'll need to choose one or the other...
Did you get any love with this post Flex?
AB, as cuppa pointed out no one really wants a custom solution. My unit did have the features like automatic switch over and under/ over voltage shut off, but for less than $10 off eBay for a simpler 12V -> 6V converter my solution wasn't worth it for most.
Our simple solution when remote and water is very precious is to recycle the water and get two showers with only 10litres of water.
Modified the van shower adding an additional 12V pressure pump fitted NRV's prevent shower water polluting van supply.
Heat up a bucket of water over the camp fire tip it into the shower recess start pump and you can shower until the water runs cold.
Dump when finished and you're ready for following day.