I have one of these ...
http://www.2daloo.com/rainman.htmlIn essence it's the same thing as the companion ... which I believe is a copy of the rainman unit.
It works brilliantly!! Heat by the fire, on gas, or under a black plastic garbage bag in the sun. The rainman has an advantage of having the shower rose free standing so you've both hands left free to wash with and don't need some way of holding the shower rose up in the air. It's around 12L which is enough for me and mrs to have a good shower each ... Flow is adjusted by how far the rose is angled (off to full).
Neither are cheap, but you get what you pay for, and the RM afaic is a for life purchase. The companion is in the same league. You won't be sorry if you buy one (does the companion have pump spares available ? .. The RM does)
The other advantage is no hoses, buckets, wires, 12v power, etc .... It's so simple she, or mr Junior can organize a hot shower all by themselves without needing help from me
Kev..