1. go down to your local scrap yard
2. pick up a large quantity of old foam fire extinguishers for about a quid each, avoid rusty ones
3. atleast one of them should have a thread close enough to a standard bsp thread for it to work, jam a fitting in and araldite in place

tips: bring a 1/2 bsp fitting with you and compare the od of where the valve enters the cylinder to the 1/2. this will give you a good idea if the extinguisher will be suitable. also avoid the really cheap disposable ones (easy to spot, they have plastic valves) as there is no screw part in them.