After a bit of trial and error, and lots of leaking bottles we settled on using old rubber Tyre inners to wrap around the bottle before inserting it into the hole in the iron and sealing it with a roofing sealant. We then filled the bottles with purified water and about 10ml of bleach, and used a silicon sealant around the bottle top.
These were then ready to be inserted into a roof of a local manyatta (maasai mud house).