Thanks Matt, that's pretty much what I'm after.
Awhile ago when I checked fleabay, there was nothing like it.
More than 15l would be good too though, so I might see the local fabricators and see what they can do as well.
Unfortunately I don't have any space for water in the tub, as I chose to use the space either side for other things and the drawers go pretty much all the way to the headboard.