The original billy cans were just cleaned out bully-beef tins with fencing wire handles attached. Some bright spark decided lids would be a good idea — and the technology was perfected.
So, choosing a billy isn’t too complicated — make sure the can is the right size for your needs and look for good quality construction (you don’t want the handle breaking off when you’re hoisting a billy full of boiling water). But, most importantly of all (only because it is the thing most often overlooked), check how well the lid fits.
The simple purpose of the lid is to keep some of the heat in and all of the ash out — so it does not need to fit firmly. A tight fitting lid is a dangerous liability — struggling to remove a billy lid is bound to get you in hot water (ha). You should be able to remove the lid from an empty billy with one hand – if you can pick up the whole billy with the lid handle, then the lid’s too tight.